T-type Ca²⁺ channel modulation by otilonium bromide

Abstract

Antispasmodics are used clinically to treat a variety of gastrointestinal disorders by inhibition of smooth muscle contraction. The main pathway for smooth muscle Ca²⁺ entry is through L-type channels; however, there is increasing evidence that T-type Ca²⁺ channels also play a role in regulating contractility. Otilonium bromide, an antispasmodic, has previously been shown to inhibit L-type Ca²⁺ channels and colonic contractile activity. The objective of this study was to determine whether otilonium bromide also inhibits T-type Ca²⁺ channels. Whole cell currents were recorded by patch-clamp technique from HEK293 cells transfected with cDNAs encoding the T-type Ca²⁺ channels, Ca_V3.1 (α1G), Ca_V3.2 (α1H), or Ca_V3.3 (α1I) alpha subunits. Extracellular solution was exchanged with otilonium bromide (10⁻⁸ to 10⁻⁵ M). Otilonium bromide reversibly blocked all T-type Ca²⁺ channels with a significantly greater affinity for Ca_V3.3 than Ca_V3.1 or Ca_V3.2. Additionally, the drug slowed inactivation in Ca_V3.1 and Ca_V3.3. Inhibition of T-type Ca²⁺ channels may contribute to inhibition of contractility by otilonium bromide. This may represent a new mechanism of action for antispasmodics and may contribute to the observed increased clinical effectiveness of antispasmodics compared with selective L-type Ca²⁺ channel blockers.

antispasmodics have been used clinically for many years to treat gastrointestinal disorders thought to be associated with increased contractility (4, 8, 11, 24). The mechanisms of action of this class of drugs are diverse and not fully understood. A key mechanism of action is thought to be inhibition of smooth muscle contractility, hence their class name. Together with their antimuscarinic effects, several have been shown to inhibit L-type Ca²⁺ channels (1, 15, 29). Entry of Ca²⁺ into gastrointestinal smooth muscle through L-type Ca²⁺ channels is required for initiation of gastrointestinal contractions. However, selective L-type Ca²⁺ channel blockers such as nifedipine do not have the same clinical effectiveness as the antispasmodic class of drugs, suggesting other mechanisms of action.

One antispasmodic is otilonium bromide (OB), a quaternary ammonium salt used clinically for the treatment of irritable bowel syndrome (3), a functional disorder of the gastrointestinal tract with high prevalence in Western countries (19). The etiology of irritable bowel syndrome is incompletely understood. Major complementary hypotheses include afferent neuronal hypersensitivity, postinfective hypersensitivity, abnormal bacteria/host interactions, small intestinal bacterial overgrowth, psychosocial factors, and smooth muscle hypercontractility (9). The latter, documented in motility studies, together with the presence of crampy abdominal pain in patients, form the basis for the treatment of irritable bowel syndrome by smooth muscle relaxants. OB can inhibit spontaneous intestinal contractile activity through several mechanisms (3, 4, 14, 26) including block of L-type Ca²⁺ channel currents in smooth muscle cells from human jejunum and rat colon, in the latter leading to an abolishment of muscle contractions (16, 29). L-type Ca²⁺ channels are voltage dependent, and opening requires a depolarization of the membrane potential. The phasic depolarization required to allow L-type Ca²⁺ channels to open in gastrointestinal smooth muscle cells is generated by interstitial cells of Cajal (ICC) and then propagated through the ICC network to smooth muscle cells.

Increasing evidence points toward a role for Ca²⁺ channels in propagation of the intestinal slow wave and specifically for a role for T-type-like Ca²⁺ channels (6, 13, 25, 33). T-type-like Ca²⁺ channels are expressed in gastrointestinal smooth muscle cells and ICC (6, 12). Inhibition of T-type Ca²⁺ channels in the small intestine is postulated to disrupt propagation of the slow wave to smooth muscle, causing a decrease in smooth muscle contractility. The aim of this study was to determine the effect of OB on T-type Ca²⁺ channels Ca_V3.1, Ca_V3.2, and Ca_V3.3 exogenously expressed in HEK 293 cells.

METHODS

Transfection of HEK cells.

Construction of the expression vectors containing the genes encoding T-type Ca²⁺ channels (older nomenclature used to identify alpha subunit in parentheses) Ca_V3.1 (α1G), Ca_V3.2 (α1H), and Ca_V3.3 (α1I) used in this study has been described previously (2, 7, 23). Each of the expression vectors along with green fluorescent protein, pEGFP-C1 (Clontech, Palo Alto, CA), were transiently cotransfected into HEK293 cells (American Type Culture Collection, Manassas, VA) using Lipofectamine 2000 Reagent (Invitrogen, Carlsbad, CA). Transfected HEK cells were cultured for 2 days prior to study.

Electrophysiology.

Whole cell currents were recorded by standard patch-clamp technique from transfected cells identified by fluorescent microscopy (28, 29). Patch-clamp electrode glass Kimble KG-12 was pulled on a P-97 puller (Sutter Instrument, Novato, CA). Electrodes were coated with R6101 (Dow Corning, Midland, MI) and fire polished to a final resistance of 3–5 MΩ. Currents were amplified, digitized, and processed by use of an Axopatch 200B amplifier, a Digidata 1322A, and pCLAMP 9 software (Molecular Devices, Union City, CA). For Ca_V3.1 and Ca_V3.2, cells were stepped from a holding voltage of −100 mV to −90 through +35 mV in 5-mV intervals for 400 ms each. Start-to-start time was 1 s. Whole cell records from Ca_V3.1 and Ca_V3.2 were sampled at 10 kHz and filtered at 4 kHz with an eight-pole Bessel filter. Sampling rates of data for Ca_V3.1 and Ca_V3.2 were reduced (decimated) fivefold to 2 kHz during analysis. Ca²⁺ currents completely inactivated before 400 ms in HEK cells transfected with Ca_V3.1 or Ca_V3.2 but not Ca_V3.3. Therefore, the protocol was modified slightly for Ca_V3.3; cells were stepped from a holding voltage of −100 mV to −90 through +30 mV in 10-mV intervals for 2 s each. Start-to-start time was 4 s. Whole cell records from Ca_V3.3 were sampled at 2 kHz and filtered at 1 kHz with an eight-pole Bessel filter. The sampling rate of data for Ca_V3.3 was not reduced during analysis, and 70–80% series resistance compensation (lag of 60 μs) was applied during all recordings.

For use- and time-dependent block experiments, HEK cells transfected with T-type Ca²⁺ channels were held at −120 mV and repeatedly stepped to −30 mV (a voltage positive of peak inward current, chosen to ensure full open probability) for 15 ms (Ca_V3.1, Ca_V3.2) or 30 ms (Ca_V3.3) at 2 or 40 s start-to-start intervals. Total elapsed time per record was 200 s. Eight subsweeps of P/N leak subtraction were applied to each trace during recording.

Measurement of contractions from human colonic smooth muscle.

The Institutional Review Board approved the use of specimens of human colonic tissue resected for nonobstructive colon cancer. Normal sigmoid colon at least 10 cm away from the tumor was harvested directly into chilled F12 buffer solution (F-12: GIBCO, Invitrogen, 14 mmol/l NaHCO₃; pH 7.35) and transported to the laboratory within 20 min. After the mucosa was removed, a strip of muscle (2 × 10 mm) was cut with the long axis of the muscle strip parallel to the circular muscle layer. One end of the muscle strip was pinned down to the Sylgard-coated floor of a recording chamber and the other end of the strip was tied with a string and attached to a force transducer. A stretch was applied to the strip, which extended its length to 1.5 to 1.6 times its original length. The recording chamber was continuously perfused with oxygenated normal Krebs solution (NKS) at 37°C and at a rate of 3 ml/min. Recording started ∼2 h after the muscle strip was placed into the recording chamber. The composition of the NKS was (in mM) 137.4 Na⁺, 5.9 K⁺, 2.5 Ca²⁺, 1.2 Mg²⁺, 124 Cl⁻, 15.5 HCO₃⁻, 1.2 H₂PO₄⁻, and 11.5 glucose. It was continuously bubbled with 97% O₂-3% CO₂, and maintained at pH 7.4. Values are given as means ± SE and a P value of <0.05 is considered significant.

Drugs and solutions.

The pipette solution contained (in mM) 145 Cs⁺, 35 Cl⁻, 5 Na⁺, 5 Mg²⁺, 2 EGTA, 5 HEPES, 125 methanesulfonate. pH was adjusted to 7.0 with CsOH. The bath solution contained normal Ringer solution (in mM) 149.2 Na⁺, 4.74 K⁺, 159 Cl⁻, 2.54 Ca²⁺ buffered with 5 mM HEPES. pH was adjusted to 7.4 with NaOH. Osmolality of solutions was 300 mmol/kg. Chemicals and mibefradil (3 μM) were purchased from Sigma Chemical (St. Louis, MO). OB was obtained from Menarini Ricerche, Florence, Italy. For washout experiments, 6–15 ml of normal Ringer solution was applied over the course of 3–8 min. The median time for all washout experiments was 5.1 min (n = 20). After a seal and access were established, control recordings were taken in NaCl Ringer solution for 2–5 min until T-type Ca²⁺ current reached a steady value. Next, each cell was exposed for 30–120 s with Ringer solution containing increasing concentrations of OB (0 M OB control rinse followed by 10⁻⁸ to 10⁻⁵ M OB).

Data analysis.

Data were analyzed via Clampfit and Excel (Microsoft, Redmond, WA). Current-voltage (I-V) curves plot peak inward current vs. voltage steps. Data were normalized by using the formula I_norm = 100(I_v)/(I_max), where I_norm is the normalized peak inward current at a particular voltage sweep, I_v is the current at each voltage sweep, and I_max is the maximum peak inward current from the set of traces (usually the peak inward current at −50 mV). Dose-responses (Fig. 1–3) were fitted with a logistic three-parameter curve (Hill equation): y = a/[1+(x/x₀)^b], where a is the y-intercept (∼100%), b is the slope, and x₀ is the IC₅₀ value. Time to peak (Fig. 6) was measured from the start of each pulse to the time of peak inward current. Inactivation kinetics (Fig. 6) were fitted by a two-term weighted exponential function: $f(t)=K_0(f_1{e}^{-t/{\tau }_1}+f_2{e}^{-t/{\tau }_2})+\text{C}$, where τ₁ and τ₂ are time constants of inactivation and activation, respectively. Currents recorded in the presence of OB were vertically scaled to visually compare the kinetics of partially blocked currents to those from control (Fig. 6). Time to peak and time constant of inactivation are measurements of time, and kinetic measurements from scaled currents were confirmed to be the same as unscaled within 0.01 ms. Significance was determined via GraphPad software (La Jolla, CA) by nonparametric repeated-measures ANOVA with Friedman posttest (Figs. 1–3, 5) or Student's t-test (Figs. 6–7). A P value of < 0.05 was considered significant.

Fig. 1.

Otilonium bromide (OB) blocks T-type Ca²⁺ channel subunit Ca_V3.1 (α1G) expressed in HEK293 cells. A: representative whole cell T-type Ca²⁺ channel currents recorded in NaCl Ringer solution with 0, 0.1, 1, or 10 μM OB. Inset: OB molecular structure. B: current-voltage relationships of T-type Ca²⁺ currents at each concentration of OB were normalized to the maximum peak inward current of the control record (0 M OB). C: summary of normalized peak inward Ca²⁺ currents (n = 7 cells exposed to the entire sequence of concentrations, *P < 0.05 by nonparametric repeated-measures ANOVA with Friedman posttest). The IC₅₀ value of OB for Ca_V3.1 was 774 ± 109 nM (dotted line).

Fig. 2.

OB blocks T-type Ca²⁺ channel subunit Ca_V3.2 (α1H) expressed in HEK293 cells. A: representative whole cell T-type Ca²⁺ channel currents recorded in NaCl Ringer solution with 0, 0.1, 1, or 10 μM OB. B: current-voltage relationships of T-type Ca²⁺ currents at each concentration of OB were normalized to the maximum peak inward current of the control record (0 M OB). C: summary of normalized peak inward Ca²⁺ currents (n = 6 cells exposed to the entire sequence of concentrations, *P < 0.05 by nonparametric repeated-measures ANOVA with Friedman posttest). The IC₅₀ value of OB for Ca_V3.2 was 1,070 ± 269 nM (dotted line).

Fig. 3.

OB blocks T-type Ca²⁺ channel subunit Ca_V3.3 (α1I) expressed in HEK293 cells. A: representative whole cell T-type Ca²⁺ channel currents recorded in NaCl Ringer solution with 0, 0.1, 1, or 10 μM OB. B: current-voltage relationships of T-type Ca²⁺ currents at each concentration of OB were normalized to the maximum peak inward current of the control record (0 M OB). C: summary of normalized peak inward Ca²⁺ currents (n = 7 cells exposed to the entire sequence of concentrations, *P < 0.05 by nonparametric repeated-measures ANOVA with Friedman posttest). The IC₅₀ value of OB for Ca_V3.3 was 451 ± 90 nM (dotted line).

Fig. 6.

OB (3 μM) delays activation and inactivation kinetics. Left: equally scaled traces obtained by stepping from −100 to −50 mV, showing Ca_V3.1 (A–B), Ca_V3.2 (C), or Ca_V3.3 (D) currents in response to sham rinses (A) or exposures up to 3 μM extracellular OB (B–D). Traces are from the same experiments shown in Figs. 1A, 2A, and 3A. Middle: time from start of pulse to point of maximum peak inward current (*P < 0.05, control to 3 μM OB; Ca_V3.1, n = 6; Ca_V3.2, n = 5; Ca_V3.3, n = 6). Right: time constant of inactivation vs. step voltage (*P < 0.05, control to 3 μM OB; Ca_V3.1, n = 6; Ca_V3.2, n = 6; Ca_V3.3, n = 6).

Fig. 5.

Block of T-type Ca²⁺ channels by OB is reversible. Left: representative traces of maximum peak inward Ca²⁺ currents from Ca_V3.1 (A), Ca_V3.2 (B), or Ca_V3.3 (C) in the presence of 0 (control), 3, or 0 (washout) μM OB. OB (3 μM) inhibited the Ca_V3.1, Ca_V3.2, and Ca_V3.3 current traces by 83, 67, and 92%, respectively, during steps from −100 to −40 mV. Washout recovered 100, 92, and 94% of their respective control traces. Right: peak Ca²⁺ currents normalized to same-cell pre-OB controls (*P < 0.05 by nonparametric repeated-measures ANOVA with Friedman posttest; Ca_V3.1, n = 6; Ca_V3.2, n = 6; Ca_V3.3, n = 8).

Fig. 7.

Mibefradil (3 μM) decreases the frequency of contractions in human colon. Circular muscle layer contractions recorded from a normal human sigmoid colonic muscle strip before (A) and during (B) infusion of 3 μM mibefradil.

RESULTS

Cell capacitances were similar between the cells that expressed Ca_V3.1 (13.0 ± 2.2 pF, n = 13), Ca_V3.2 (11.4 ± 1.6 pF, n = 12), and Ca_V3.3 (9.5 ± 1.7 pF, n = 13) (P > 0.05). The average peak current density of Ca_V3.2 (−22.4 ± 5.3 pA/pF, n = 12) was significantly less than either Ca_V3.1 (−60.3 ± 13.3 pA/pF, n = 13, P < 0.05) or Ca_V3.3 (−60.9 ± 14.5 pA/pF, n = 13, P < 0.05).

OB blocks T-type Ca²⁺ channel alpha subunits expressed in HEK293 cells with a lower concentration for Ca_V3.3 than Ca_V3.1 or Ca_V3.2.

Ca_V3.1 current inactivated by 200 ms, peaked at approximately −50 mV step voltage, and was significantly blocked by 82.6 ± 2.2% at 3 μM and 96.7 ± 0.7% at 10 μM OB (Fig. 1, n = 6, P < 0.05). Ca²⁺ current through Ca_V3.2 completely inactivated before 400 ms, peaked at about −45 mV step voltage, and was significantly blocked by 76.0 ± 3.6% at 3 μM and 87.4 ± 3.2% at 10 μM OB (Fig. 2, n = 6, P < 0.05). Ca_V3.3 current completely inactivated by 1 s, peaked at −50 to −40 mV step voltage, and was significantly blocked by 73.2 ± 4.2% at 1 μM, 91.6 ± 2.2% at 3 μM, and 96.3 ± 1.4% at 10 μM OB (Fig. 3, n = 7, P < 0.05).

Dose-response curves generated from the above data (Figs. 1C, 2C, and 3C) demonstrated that OB inhibited T-type Ca²⁺ channel subunit Ca_V3.3 at lower concentrations than Ca_V3.1 or Ca_V3.2 (Table 1). The corresponding IC₅₀ value for Ca_V3.3 (451 ± 90 nM, slope 1.23 ± 0.07, n = 7) was significantly less than Ca_V3.1 (774 ± 109 nM, n = 6, P < 0.05) or Ca_V3.2 (1,070 ± 269 nM, n = 6, P < 0.05). Although the IC₅₀ values for Ca_V3.1 and Ca_V3.2 were similar, their respective slope constants were significantly different (1.54 ± 0.14 vs. 1.11 ± 0.08, n = 6 each, P < 0.05).

Table 1.

IC₅₀ values of otilonium bromide inhibition of L- (Ca_V1.2) and T-type (Ca_V3.1, Ca_V3.2, Ca_V3.3) Ca²⁺ and Na⁺ (Na_V1.5) channels

Channel	IC50, μM	Slope
CaV1.2	2.3 ± 0.5	1.7 ± 0.4
CaV3.1	0.8 ± 0.1	1.5 ± 0.1
CaV3.2	1.1 ± 0.3	1.1 ± 0.1
CaV3.3	0.4 ± 0.1	1.2 ± 0.1
NaV1.5	8.8 ± 2.4	4.0 ± 3.2

Ca_V1.2 data were taken from literature (29).

Time course for OB block.

Cells were incubated with increasing concentrations of OB. IC₅₀ values may be skewed if block did not reach a plateau at each concentration or if the block were use dependent. Therefore, we tested block of T-type Ca²⁺ currents opening in response to steps to −30 mV at two different frequencies, 0.5 and 0.025 Hz (Fig. 4). OB (3 μM) blocked T-type Ca²⁺ currents down to a plateau by 30–60 s at 0.5 Hz and 80 s at 0.025 Hz (Fig. 4, right), suggesting that the block was not dependent on channel openings because the plateau at 0.5 Hz was reached after more pulses than at 0.025 Hz. Time to plateau (Fig. 4) also was shorter than sequential rinse intervals (Fig. 1–3), suggesting that the time period between rinses was adequate.

Fig. 4.

Block by OB is not use dependent. T-type Ca²⁺ channels Ca_V3.1 (A), Ca_V3.2 (B), or Ca_V3.3 (C) were stepped to −30 mV from a holding potential of −120 mV once every 2 s (0.5 Hz, left) or 40 s (0.025 Hz, middle) in the presence of 3 μM OB. Traces at 0 (solid lines) and 200 s (dotted lines) are shown. Right: percent block of T-type Ca²⁺ currents vs. time. Peak Ca²⁺ currents pulsed to −30 mV at 0.5 Hz (solid symbols, 1 of every 4 data points shown) or 0.025 Hz (open symbols, all data points shown) were normalized to the trace at time = 0 in the absence (circles) or presence (triangles) of OB (3 μM).

Block of T-type Ca²⁺ channels by OB is reversible.

To determine whether inhibition of T-type Ca²⁺ channels by OB was reversible, we recorded current after washing off the drug (Fig. 5). T-type Ca²⁺ currents could not be recorded from HEK 293 cells for longer than 1–2 min in the presence of 10 μM OB. Therefore, we applied 3 μM before washout. Following significant block of Ca_V3.1 (79.2 ± 3.0% block, n = 6, P < 0.05), Ca_V3.2 (67.1 ± 3.9% block, n = 6, P < 0.05), and Ca_V3.3 (79.1 ± 5.4% block, n = 8, P < 0.05) by OB (3 μM), peak Ca²⁺ currents for all three T-type channels returned to levels at or below baseline (Ca_V3.1: 83.2 ± 9.6% of baseline current, n = 6, P > 0.05; Ca_V3.2: 80.0 ± 7.2%, n = 6, P > 0.05; Ca_V3.3: 74.2 ± 6.1%, n = 8, P > 0.05) (Fig. 5) within 5 min.

OB slows the activation and inactivation kinetics of Ca_V3.1.

T-type Ca²⁺ currents at the six increasing concentrations of OB shown in Fig. 1 were vertically scaled to match the baseline level of peak inward current of 0 M controls and then analyzed for change in activation times (Fig. 6, middle) or inactivation time constants (Fig. 6, right). To control for any changes in kinetics induced by time or rinses, we carried out control experiments with sham rinses of control solution. In response to rinsing the bath up to six times with drug-free Ringer solution, Ca_V3.1 activated the same or faster and inactivated faster at all voltage steps tested (Fig. 6A). At the 3 μM dose of OB, Ca_V3.1 was slower to both activate and inactivate at voltage steps from −100 to −60 through 0 mV (Fig. 6B; n = 6, P < 0.05). In contrast, 3 μM OB changed activation and inactivation channel kinetics of Ca_V3.2 or Ca_V3.3 at only select voltage steps (Fig. 6, C and D; n = 6 each, P < 0.05 by Student's t-test where indicated by asterisk).

Mibefradil reduces frequency of contractions in human colon.

We determined the potential contribution of T-type Ca²⁺ channels to smooth muscle contractions in human colon. Both high-frequency, low-amplitude contractions and low-frequency, high-amplitude contractions were observed in muscle strips (Fig. 7). In NKS the frequency was 6.03 ± 1.33 min⁻¹ for the high-frequency, low-amplitude contractions and 0.73 ± 0.18 min⁻¹ for the low-frequency, high-amplitude contractions (n = 4). Twenty minutes after application of mibefradil (3 μM), the frequency of the high-frequency, low-amplitude contractions was 4.73 ± 0.78 min⁻¹ (P = 0.068, Fig. 7) whereas the frequency of the low-frequency, high-amplitude contractions decreased to 0.46 ± 0.06 min⁻¹ (P = 0.05, Fig. 7).

DISCUSSION

The main finding of this study was that the antispasmodic drug OB inhibited T-type Ca²⁺ channels at concentrations that are clinically significant, therefore representing an additional potential mechanism of action of this drug. OB therefore appears to have diverse mechanisms of action, targeting a variety of colonic channels and receptors.

T-type Ca²⁺ channels have been identified in a wide array of tissues, including neurons, cardiac pacemaking cells, and smooth muscle cells from bronchial, vascular, uterine, and gastrointestinal tissues (34). T-type Ca²⁺ channels may play a role in electrophysiological pacing in tissues when present, including the gastrointestinal tract.

In the gastrointestinal tract of various species, T-type or T-type-like Ca²⁺ currents have been recorded from stomach (30), small intestine (6, 27), and colon (10, 31, 32), the latter representing the predominant tissue affected by OB. The T-type Ca²⁺ channel family is made up of three genes, CACNA1G, CACNA1H, and CACNA1I, which respectively encode the T-type Ca²⁺ channels Ca_V3.1, Ca_V3.2, and Ca_V3.3 (also known as α1G, α1H, and α1I) (22). Cloned “in silico” by human brain EST tags, Ca_V3.1 in the gastrointestinal tract has been confirmed by Northern blot (20) and RT-PCR (21). In mouse, nifedipine- and cadmium-resistant, mibefradil-sensitive currents have been identified in jejunum myocytes, and loss of Ca_V3.2 expression decreased the upstroke and slow wave frequency of slow waves by ICC (6). The presence of Ca_V3.3 in gastrointestinal smooth muscle is not clear, with evidence both for (21) and against its expression (6).

The main findings of this study were that OB reversibly blocked the gastrointestinal T-type Ca²⁺ channels Ca_V3.1 (α1G) and Ca_V3.2 (α1H). OB was also effective at blocking Ca_V3.3 (α1I), with a lower IC₅₀ suggesting that OB had a greater affinity to this T-type channel subunit. The IC₅₀s for all three channels were within the range (∼1–10 μM) of the known concentration of OB in gastrointestinal smooth muscle (5). These values are significantly less than the IC₅₀ values for two other gastrointestinal smooth muscle channels when expressed heterologously, the L-type Ca²⁺ channel Ca_V1.2 [alpha subunit α1C with auxiliary subunit β2, 2.3 ± 0.5 μM (29)] and Na⁺ channel Na_V1.5 (α pore encoded by SCN5A, 8.8 ± 2.4 μM, Table 1). Overall, OB blocks channels at IC₅₀ values in the following order: Ca_V3.3 < Ca_V3.1 = Ca_V3.2 < Ca_V1.2 < Na_V1.5. Therefore, the block observed in this study may be clinically significant. This statement is supported by the effect of mibefradil on human sigmoid colon contractile activity, although the effect of mibefradil on the complex motor patterns of the human colon will require further study.

OB appears to block T-type Ca²⁺ channels at doses in a similar range as the established T-type Ca²⁺ blocker, mibefradil. IC₅₀ values for the block of transfected Ca²⁺ channels by mibefradil for Ca_V3.1 (0.27 μM), Ca_V3.2 (0.14 μM), Ca_V3.3 (0.29 μM) (17, 28) were close to the IC₅₀ values of OB reported here (Table 1). However, mibefradil does not appear to be as specific for T-type Ca²⁺ channels as OB with a threefold selectivity for mibefradil on T-type channels over Na_V1.5 channels compared with 9-fold for otilonium (18, 28).

In conclusion, OB inhibits T-type Ca²⁺ channels at clinically relevant concentrations. This effect on T-type Ca²⁺ channels is likely limited to gastrointestinal T-type Ca²⁺ channels as OB is poorly absorbed, with plasma levels at least 1,000 times lower than those reached in the colonic wall.(5) Therefore, inhibition of gastrointestinal T-type Ca²⁺ channels may contribute to the inhibitory effects of OB on smooth muscle contractility. Whether this effect is specific for OB or is a general property of antispasmodics will require future investigation.

GRANTS

This research was supported by National Institutes of Health grants DK52766 and DK57061 (G. Farrugia) and NS38691 (E. Perez-Reyes) and an unrestricted grant from Menarini Ricerche.