Characteristics of Ca Currents in Rabbit Basilar Arterial Smooth Muscle Cells

Abstract

In order to determine the exact nature of Ca channels involved in various cerebrovascular contractile behaviour including vasospasm, we performed experiments to identify and characterize the types of Ca channels in rabbit basilar arterial smooth muscle cells by using kinetic and pharmacologic tools. Sngle smooth muscle cells were enzymatically isolated from rabbit basilar artery. Single cells were voltageclamped, and membrane currents were recorded using the whole-cell configuration of patch clamp technique. The measured cell capacitance (Cm) was 19.2 ± 0.65 pF ( n 21) and input resistance (Rmpu, ) was 2.04 ± 0.12 Gn(n 12), These passive membame properties are similar to other cerebraovascular smooth muscle cells. Inward Ca2 ' -channel current was recorded. Replacement of external Ca2+( 2 mM) with Ba" (10 mM) increased the amplitude of the current and did not shift the I-V relationship for lBa in comparison with that for Ie.. Changing the holding potential from -SO to -40 mV decreased the current amplitude but did not shift the voltage dependence. No detectable low-threshold, rapid inactivating inward current was observed. Steady-state activation and inactivation curves for lea(V, 210c') -4.4 mV; V, 2(inac,) -22.3 mV) and lBa(v, 2(oc') -7.5 mV; VI 2 {Inact } -20.5 mV) were determined. The theoritical 'window current's amplitude was calculated for lea and lBa• Calcium channel current was almost completely inhibited by l,uM nicardipine and enhanced by Bay K 8644, suggesting this is carried by 'L-type' but not by 'T-type' calcium channel. Bay K 8644 significantly shifted activation curve to the negative potential. Both 8-br-cAMPW.l ~ 1 mM) and 8-br-cGMP( 0.1 ~ 1 mM), a membrane permeable cyclic nucleotides, decreased the current amplitude. From the above results, it is suggested that only 'L-type' Ca-current(ICa_d exists in rabbit basilar arterial smooth muscle cells.