Figure 1.

Muscarinic inhibition and stimulation of the L-type Ca²⁺ current. Top panel: Time course of changes in the magnitude of the L-type Ca²⁺ current recorded from an isolated guinea-pig ventricular myocyte following exposure to the muscarinic receptor agonist ACh in the presence of a submaximally stimulating concentration of the β-adrenergic receptor agonist isoprenaline (Iso). Currents were elicited by applying 100 ms depolarizing voltage clamp steps to a test potential of 0 mV following a 40 ms conditioning pulse to −30 mV from a holding potential of −80 mV. The Ca²⁺ current was measured as the absolute magnitude of the peak inward current elicited during the test pulse. Muscarinic inhibition of the β-adrenergic response (I) is observed in the presence of ACh. Washout of ACh in the continued presence of β-adrenergic stimulation reveals the rebound stimulatory response (S) to muscarinic receptor activation. Bottom panel: Current traces recorded at time points indicated in top panel. (1) Steady-state response to submaximally stimulating concentration of Iso, (2) steady-state inhibitory effect of ACh in the presence of Iso, and (3) peak rebound stimulatory effect following washout of ACh in the continued presence of Iso. Changes in the steady-state pre- and post-test pulse currents are due to parallel activation of the cAMP regulated, time-independent Cl⁻ current. The Cl⁻ current did not affect measurement of the Ca²⁺ current at the test potential since the Cl⁻ equilibrium potential was set at 0 mV. Further experimental details are as described in Belevych et al. (2001).