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Department of Veterinary Science, Faculty of Agriculture, Gifu University, Japan.
1. The effect of muscarinic receptor stimulation on voltage-gated calcium channel currents was examined in whole-cell voltage-clamped smooth muscle cells of the guinea-pig ileum. 2. In cells voltage clamped at -60 mV and in which calcium channel currents (ICa) were elicited repeatedly by depolarizing pulses (25 ms duration, 0.25 Hz frequency) to 0 mV, carbachol (CCh, 10 microM) induced an inward current (ICCh) and were suppressed ICa, in a biphasic manner; an initial transient component was followed by a more sustained one. 3. A calcium channel current (IBa), when Ba2+ was used as a charge carrier, was also suppressed by CCh in a biphasic manner, as with ICa. The sustained phase of the IBa suppression was significantly smaller than that of the ICa suppression, suggesting that Ca2+ entry exerts a potentiating effect on the current suppression. 4. CCh had little or no effect on calcium channel currents (ICa and IBa) in cells dialysed with a pipette solution containing EGTA (20 mM). 5. Inclusion of GDP-beta-S (1 mM) in the pipette solution abolished ICCh and the suppression of IBa. With GTP-gamma-S (10 microM) in the pipette, the sustained phase of the IBa suppression remained almost unchanged even after removal of CCh. 6. Pretreatment with 2 micrograms ml-1 pertussis toxin (PTX), which abolished ICCh, did not change noticeably the initial transient and sustained phases of IBa suppression. 7. Neomycin (100 microM) or heparin (5 mg ml-1) in the pipette each abolished the initial transient component of ICCh as well as the initial transient phase of IBa suppression. 8. The biphasic effect of CCh on IBa was observed in the presence of either staurosporine (1 microM) or 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (100 microM). Phorbol 12-myristate 13-acetate and phorbol 12,13-dibutyrate (up to 10 microM) had no inhibitory effect on ICa and IBa. 9. The results suggest that stimulation of the muscarinic receptor causes a biphasic suppression of the voltage-gated calcium channel currents through a PTX-insensitive G protein in guinea-pig ileal smooth muscle cells. The initial transient phase may be brought about by the release of Ca2+ from internal storage sites, and the sustained phase by a Ca(2+)-dependent mechanism which is independent of the phosphatidylinositol pathway.
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