Low‐voltage‐activated calcium current in rat aorta smooth muscle cells in primary culture.

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Abstract
1. Electrical and pharmacological properties of the low‐voltage‐activated Ca2+ current (ICa, LVA) in rat aorta smooth muscle cells (SMC) in primary culture were examined, particularly in comparison with the high‐voltage‐activated Ca2+ current (ICa, HVA). Both types of Ca2+ currents were recorded in external solution containing 20 mM‐Ca2+, using the whole‐cell voltage‐clamp technique. 2. ICa, LVA was evoked by step depolarizations to potentials more positive than ‐60 mV from a holding potential of ‐100 mV, and reached a peak in the current‐voltage (I‐V) relationship around ‐30 mV. ICa, HVA was activated at ‐20 mV, and reached a peak at +20 mV. 3. The intracellular dialysis of 5 mM‐F‐ irreversibly suppressed ICa, HVA, with time, while it has little effect on the ICa, LVA. The ICa, LVA could be separated from the ICa, HVA by either selecting the holding and test potential levels or by perfusing intracellularly with F‐. 4. The ratio of peak amplitude of Ba2+, Sr2+ and Ca2+ currents in the respective I‐V relationship was 1.6:1.2:1.0 for high‐voltage‐activated Ca2+ channels and was 1.0:1.4:1.0 for low‐voltage‐activated ones. 5. The inactivation phase of ICa, HVA was fitted by a sum of double‐exponential functions, the time constants of which were larger when the current was carried by Ba2+ than by Ca2+. The inactivation time course of ICa, LVA was fitted by a single‐exponential function, and the time constant was practically the same when the current was carried by Ba2+ or by Ca2+. Activation and inactivation processes of ICa, LVA were potential‐dependent. 6. The steady‐state inactivation curve of ICa, LVA was fitted by the Boltzmann equation, having a mid‐potential of ‐80 mV and a slope factor of 5.0. The recovery time course from steady‐state inactivation was fitted by a sum of two exponential functions. The time constants of the faster phase were 230 and 380 ms, and those of slower phase were 2.8 and 1.8 s at the repolarization potentials of ‐120 and ‐100 mV, respectively. 7. The amplitude of ICa, LVA depended on the external Ca2+ concentration ([Ca2+]o), approaching saturation at 95 mM [Ca2+]o. 8. Various polyvalent cations blocked both types of Ca2+ current reversibly in the order (IC50 in M): La3+ (8 x 10(‐8)) greater than Cd2+ (6 x 10(‐6)) greater than Ni2+ (1 x 10(‐5)) greater than Zn2+ (2 x 10(‐5)) for ICa, HVA, and La3+ (6 x 10(‐7)) greater than Zn2+ (3 x 10(‐5)) greater than Cd2+ (4 x 10(‐4)) greater than Ni2+ (6 x 10(‐4)) for ICa, LVA.(ABSTRACT TRUNCATED AT 400 WORDS)