Effects of isoprenaline (ISO), carbachol and phorbol ester (a stimulator of protein kinase C) on L-type Ca2+ channels in single cultured rat aortic vascular smooth muscle (A7r5) cells were examined using whole-cell voltage clamp (at room temperature 22 degrees C). 2. With 20 mmol/l Ca2+ in the bath solution and 10 mmol/l EGTA in the pipette solution, a slow ICa (L-type) current was observed in the A7r5 cell line, which was blocked by nifedipine (2 mumol/l). 3. ISO (5 mumol/l) inhibited ICa by 18.3 +/- 2.2% (P < 0.001), and carbachol (1 mumol/l) also decreased ICa by 15.0 +/- 3.2% (P < 0.01). 8-Br-cAMP (1 mmol/l) and 8-Br-cGMP (1 mmol/l) both inhibited ICa by 30.1 +/- 2.8% (P < 0.001) and 18.8 +/- 3.8% (P < 0.01), respectively. 4. Phorbol ester, 4-beta-phorbol-12, 13-dibutyrate (PDB), at 0.1-1 mumol/l, had almost no effect on ICa in most cells, but slightly potentiated (or slightly enhanced) the inhibitory effects of ISO. 5. Time decay (inactivation) of ICa consisted of two exponentials. Both the fast and slow time constants were slightly prolonged by ISO (5 mumol/l), and by carbachol (1 mumol/l); PDB (1 mumol/l) slightly shortened the fast time constant only. The half-maximum voltages of inactivation were not significantly affected by any of the agents. 6. These results suggest that the L-type ICa 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.
