Effect of dihydropyridines on calcium channels in isolated smooth muscle cells from rat vena cava

• Published in: British journal of pharmacology Vol. 105; no. 2; pp. 321 - 328
• Main Authors: Mironneau, J., Yamamoto, T., Sayet, I., Arnaudeau, S., Rakotoarisoa, L., Mironneau, C.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.1992; Nature Publishing
• Subjects: Animals; Barium - metabolism; Biological and medical sciences; Ca2+ channel; calcium; Calcium Channel Blockers - pharmacology; Calcium Channels - drug effects; Calcium Channels - metabolism; Cardiovascular system; channels; dihydropyridine; dihydropyridines; Dihydropyridines - pharmacology; effects on; Electrophysiology; In Vitro Techniques; Ion Channels - drug effects; isolated smooth muscle cells; Isradipine; Medical sciences; Muscle, Smooth, Vascular - cytology; Muscle, Smooth, Vascular - drug effects; Na+ channel; Pharmacology. Drug treatments; Rats; Receptors, Nicotinic - drug effects; Receptors, Nicotinic - metabolism; smooth muscle; Sodium Channels - drug effects; Tetrodotoxin - pharmacology; Vasodilator agents. Cerebral vasodilators; Vena cava; Venae Cavae - cytology; Venae Cavae - drug effects; Vasodilator agent; Rat; Rodentia; Calcium antagonist; Electrophysiology; Smooth muscle; Ionic channel; Binding site; Muscle contraction; In vitro; Dihydropyridine derivative; Vertebrata; Mammalia; Sodium; Animal; Circulatory system; Vena cava
• ISSN: 0007-1188; 1476-5381
• DOI: 10.1111/j.1476-5381.1992.tb14253.x

Summary 1 . Whole‐cell patch‐clamp method was applied to single smooth muscle cells freshly isolated from the rat inferior vena cava. 2 . Depolarizing pulses, applied from a holding potential of − 90 mV, activated both Na+ and Ca2+ channels. The fast Na+ current was inhibited by nanomolar concentrations of tetrodotoxin (TTX). The slow Ba2+ current (measured in 5 mm Ba2+ solution) was inhibited by Cd2+ and modulated by dihydropyridine derivatives. When the cells were held at a holding potential of −80 mV, racemic Bay K 8644 increased the Ba2+ current (ED50 = 10 nm) while racemic isradipine inhibited the current (IC50 = 21 nm). 3 . The voltage‐dependency of isradipine blockade was assessed by determining the steady‐state availability of the Ca2+ channels. From the shift of the inactivation curve in the presence of isradipine, we calculated a dissociation constant of 1.11 nm for inactivated Ca2+ channels. Scatchard plots of the specific binding of (+)‐[3H]‐isradipine obtained in intact strips incubated in 5.6 mm or 135 mm K+ solutions confirmed the voltage‐dependency of isradipine binding. 4 . Specific binding of (+)‐[3H]‐isradipine was completely displaced by unlabelled (±)‐isradipine, with an IC50 of 15.1 nm. This value is similar to the IC50 for inhibition of the Ba2+ current (21 nm) in cells maintained at a holding potential of −80 mV. 5 . Bay K 8644 had no effects on the Ba2+ current kinetics during a depolarizing test pulse. The steady‐state inactivation‐activation curves of Ba2+ current were not significantly shifted along the voltage axis. 6 . The present data suggest the existence of two distinct dihydropyridine binding sites which can be bound preferentially by agonist or antagonist derivatives.