Fig. 1.
Activation of transient secretion was sensitive to β2-adrenergic antagonism. Isolated mucosae were stimulated by epinephrine (epi) or norepinephrine (norepi) addition to the serosal bath from the standard basal condition with electrogenic secretion monitored by short-circuit current (Isc) (see methods). A: Isc was measured in adjacent mucosae during activation by epi at low and high concentration, 0.1 μM (○) and 10 μM (●), respectively. The difference between high and low concentration activation illustrated the transient component (delta, △). B: responses at 3 concentrations of either epi (●) or norepi (▲) were measured in adjacent mucosae. The resulting concentration dependences of peak ΔIsc (ΔIsc = difference between stimulated and basal Isc) were fit by Henri-Michaelis-Menton kinetics with a single binding site (dashed line; maxΔIsc, maximal ΔIsc; epiEC50 and norepiEC50, half-maximal effective concentrations for epi and norepi, respectively): epiEC50 = 2.1 ± 0.6 μM, maxΔIsc = +162 ± 15 μA/cm2, n = 4; norepiEC50 = 30.9 ± 7.5 μM, maxΔIsc = +186 ± 20 μA/cm2, n = 3. For comparison, a fit is shown (dotted gray line) using maxΔIsc together with the epiEC50 obtained previously for sustained secretion (74). C: EC50 values of the peak ΔIsc responses to epi and norepi (●) were compared with those for the sustained Isc response, all in the presence of BIIE0246 (■; Ref. 74) and the 3 β-adrenergic receptors (△, ▽, ◊; Ref. 61). D: Isc was measured during activation by epi (3 μM) for a control mucosa (●) and an adjacent mucosa treated with the β2 adrenergic receptor (β2-AdrR)-selective antagonist ICI-118551 (0.1 μM) added 20 min prior to stimulation (○). The difference between control activation and activation with ICI-118551 illustrated the transient component (delta, △). E: 3 adjacent mucosae were treated with ICI-118551 20 min prior to stimulation with epi (3 μM); a fourth adjacent mucosa served as an epi activation control. A fit to the concentration dependence of epi-activated ΔIsc (epiΔIsc; n = 4, dashed line) together with the epi activation control without antagonist (+144 ± 25 μA/cm2) provided an IC50 for ICI-118551 of 2.0 ± 0.2 nM. The calculated Kd was 4.7 ± 1.4 nM, assuming competitive antagonism. For comparison a fit is shown (dotted gray line) using the same Kd as obtained previously for sustained secretion, 131 ± 23 nM (74).