Modulation of Ca2+ channel activity by ATP metabolism and internal Mg2+ in guinea‐pig basilar artery smooth muscle cells.

1. Single smooth muscle cells were isolated from the basilar artery of the guinea‐pig and, within 10 h, inward currents through voltage‐gated Ca2+ channels were recorded using the amphotericin or conventional whole‐cell voltage‐clamp techniques. 2. In amphotericin whole‐cell recordings, bath application of 2,4‐dinitrophenol (DNP, an uncoupler of mitochondrial ATP production) induced an initial stimulation (14% increase in 5 of 11 cells) and then pronounced inhibition (50% decrease in 9 of 11 cells within 9.5 min) of voltage‐dependent Ca2+ current (I(Ca)) elicited by depolarizing to +10 mV in 1.5 mM extracellular Ca2+ solution. By contrast, inhibition of glycolysis by replacing glucose in the bath with 2‐deoxy‐D‐glucose had no effect. 3. Na+ current through Ca2+ channels (I[(Ca)(Na)]) recorded in the absence of extracellular divalent cations also responded to DNP, again with stimulation followed by inhibition of current. The stimulation of I[(Ca)(Na)] was associated with a leftward shift of the Ca2+ channel activation curve which averaged ‐9 mV. A combination of 2‐deoxy‐D‐glucose, mannoheptulose and 3‐0‐methyl‐glucose had only minor effects on I[(Ca)(Na)], whereas rotenone had an effect similar to that of DNP in six of eight cells. 4. The amplitude of I[(Ca)(Na)] in conventional whole‐cell recordings was not different from that in amphotericin whole‐cell recordings, even without ATP in the recording pipette and with metabolic poisons in the bath solution. Furthermore, attempts to dephosphorylate the Ca2+ channels in ATP‐free conditions did not prevent I[(Ca)(Na)], and a high concentration of Mg‐ATP with or without a phosphorylation‐supporting medium in the recording pipette did not increase its amplitude. 5. In the absence of ATP, Mg2+ inhibited whole‐cell I[Ca)(Na)] with a K(d) of about 100 mu M at ‐10 mV and induced a leftward shift of the Ca2+ channel activation curve. When ATP and a phosphorylation‐supporting medium were in the recording pipette the blocking effect of free Mg2+ was reduced but the shift in the Ca2+ channel activation curve was unaffected. 6. From these data it is suggested that inhibition of mitochondrial, but not glycolytic, ATP production has stimulatory and inhibitory effects on voltage‐gated Ca2+ channels of basilar artery smooth muscle cells. Effects of intracellular Mg2+ on the Ca2+ channels were modulated by ATP and mimicked the effects of metabolic poisoning by DNP. A hypothesis is discussed in which the intracellular free Mg2+ concentration may be a key factor coupling ATP production to Ca2+ channels.