Effect of depolarizing and hyperpolarizing agents on the membrane potential difference of primary cultures of rabbit aorta vascular smooth muscle cells

Abstract

Vascular smooth muscle cells of rabbit aorta were enzymatically dispersed, kept in primary culture, and studied between days 1 and 7 in a bath rinsed with Ringer-like solution at 37°C. The electrical membrane potential difference (PD) was measured with microelectrodes. The mean value of PD was −50±0.4 mV (n=53). Cromakalim (BRL 34915), 1 μmol/l and 10 μmol/l, hyperpolarized the membrane potential by 9±1 mV (n=11) and 15±1 mV (n=53) respectively. Glibenclamide (10 μmol/l) abolished the hyperpolarizing effect of cromakalim (n=6). Simultaneous addition of cromakalim and glibenclamide (both 10 μmol/l, n=11) and glibenclamide itself (10 μmol/l, n=7) had no effect on PD. In patch-clamp experiments in outside-out-oriented Ca²⁺-sensitive K⁺ channels, cromakalim increased the open probability (P _o) only slightly and only with a cytosolic Ca²⁺ activity of 1 μmol/l. In all other series cromakalim had no effect on the P _o of these channels. Forskolin (10 μmol/l) hyperpolarized PD by 6±1 mV (n=13). The nucleotides UTP, ATP and ITP (10 μmol/l) depolarized PD by 12±1 mV (n=7), 8±1 mV (n=65) and 5±1 mV (n=6) respectively. GTP, [α,β-methylene]ATP and adenosine had no significant effect. Mn²⁺ (1 mmol/l, n=18), Ni²⁺ (1 mmol/l, n=13), Co²⁺ (1 mmol/l, n=11), Zn²⁺ (1 mmol/l, n=6) and the Ca²⁺-channel blockers verapamil and nifedipine (both 0.1 mmol/l, n=6) did not attenuate the depolarization induced by 10 μmol/l ATP. Fetal calf serum (100 ml/l, n=7) depolarized PD by 11±2 mV. This effect was not abolished by nifedipine or by replacing NaCl by choline chloride. The data indicate that PD of vascular smooth muscle cells is depolarized by P₂ agonists and hyperpolarized by the K⁺-channel opener cromakalim. The effect of cromakalim is antagonized by glibenclamide. The effect of cromakalim is probably not mediated by the K⁺ channel identified in excised patches.