L, P-/Q- and T-type Ca2+ Channels in Smooth Muscle Cells from Human Umbilical Artery

Abstract

The electrophysiological and pharmacological properties of Ca²⁺ current (I_Ca) were determined by the whole-cell configuration of the patch-clamp technique in smooth muscle cells from human umbilical artery. Using 5 mM extracellular Ca²⁺, depolarizing step pulses from -60 to 50 mV from a holding membrane potential of -80 mV evoked an I_Ca which activated at membrane potentials more positive than -50 mV and exhibited a maximum current density in a range of 10-20 mV. Steady-state inactivation protocols using a V_test of 10 mV gave a voltage at one-half inactivation and a slope factor of -35.6 mV and 9.5 mV, respectively. Nifedipine (1 µM), an L-type Ca²⁺ channels antagonist, completely inhibited I_Ca, while the L-type Ca²⁺ channels agonist Bay-K 8644 (1 µM) significantly increased I_Ca amplitude. Moreover, the selective blocker of P-/Q-type Ca²⁺ channels ω-agatoxin IVA partially blocked I_Ca (about 40 % inhibition at +20 mV by 20 nM). These pharmacological results suggest that L- and P-/Q-type Ca²⁺ channels, both nifedipine-sensitive, underlie the I_Ca registered using low extracellular Ca²⁺. The presence of the P-/Q-type Ca²⁺ channels was confirmed by immunoblot analysis. When I_Ca was recorded in a high concentration (30 mM) of extracellular Ca²⁺ or Ba²⁺ as current carrier, it was evident the presence of a nifedipine-insensitive component which completely inactivated during the course of the voltage-step (75 ms) at all potentials tested, and was blocked by the T-type Ca²⁺ channels blocker mibefradil (10 µM). Summarizing, this work shows for the first time the electrophysiological and pharmacological properties of voltage-activated Ca²⁺ currents in human umbilical artery smooth muscle cells.