Modulation of L-type Ca 2+ current by isoprenaline, carbachol and phorbol ester in cultured rat aortic vascular smooth muscle (A7r5) cells

Abstract

1. 1. Effects of isoprenaline (ISO), carbachol and phorbol ester (a stimulator of protein kinase C) on L-type Ca 2+ channels in single cultured rat aortic vascular smooth muscle (A7r5) cells were examined using whole-cell voltage clamp (at room temperature 22°C). 2. 2. With 20 mmol/1 Ca 2+ in the bath solution and 10 mmol/1 EGTA in the pipette solution, a slow I Ca, (L-type) current was observed in the A7r5 cell line, which was blocked by nifedipine (2 μmol/l). 3. 3. ISO (5 μmol/l) inhibited I Ca by 18.3 ± 2.2% ( P < 0.001), and carbachol (1 μmol/1) also decreased I Ca by 15.0 ± 3.2% ( P < 0.01). 8-Br-cAMP (1 mmol/1) and 8-Br-cGMP (1 mmol/l) both inhibited I Ca by 30.1 ± 2.8% ( P < 0.001) and 18.8 ± 3.8% ( P < 0.01), respectively. 4. 4. Phorbol ester, 4-β-phorbol-12, 13-dibutyrate (PDB), at 0.1-1 μmol/l, had almost no effect on I Ca in most cells, but slightly potentiated (or slightly enhanced) the inhibitory effects of ISO. 5. 5. Time decay (inactivation) of I Ca consisted of two exponentials. Both the fast and slow time constants were slightly prolonged by ISO (5 μmol/1), and by carbachol (1 μmol/1); PDB (l μmol/l) slightly shortened the fast time constant only. The half-maximum voltages of inactivation were not significantly affected by any of the agents. 6. 6. These results suggest that the L-type I Ca current is modulated by cyclic nucleotides (cAMP and cGMP) and by PK-C stimulation, and thereby contribute to regulation of contraction of the vascular smooth muscle cells.