Ca(2+)-dependent block and potentiation of L-type calcium current in guinea-pig ventricular myocytes

1. The caged calcium compound nitr-5 has been used to investigate the response of the L-type calcium current (ICa) of guinea-pig ventricular cells to a rapid increase in the free intracellular calcium concentration ([Ca2+]i). 2. When 2 mM nitr-5 or 3 mM DM-nitrophen was loaded into cells via a patch pipette and photolysed during the decay phase of ICa, a partial block of the current developed within 75 ms. The block was reduced by increasing the pre-flash [Ca2+]i and enhanced by adding high concentrations of Ca2+ chelators to the pipette-filling solution. 3. The photolysis-induced block was not suppressed in the presence of isoprenaline, suggesting a direct action of Ca2+ on the channels rather than a mechanism involving channel phosphorylation. 4. The most prominent effect of nitr-5 photolysis was a slow potentiation of ICa. When ICa was activated at frequencies between 0.05 and 0.7 Hz with various levels of pre-flash [Ca2+]i, peak ICa was approximately doubled in amplitude following photolysis. 5. At a stimulation frequency of 0.05 Hz, when nitr-5 was the only chelator present in the pipette, the time course of the potentiation was fitted by a single exponential with a time constant (tau P) of 2.7 min. When 1 mM CaCl2 was added to the pipette-filling solution, the time course of the potentiation was slowed (tau P = 6 min), although its amplitude was unchanged. With 12 mM BAPTA (a calcium chelator) added instead of CaCl2, the response was accelerated (tau P = 1.7 min). 6. Equimolar substitution of extracellular Ca2+ with Ba2+ significantly suppressed the flash-induced potentiation. The time course of the potentiation of the barium current, IBa (tau P = 1.9 min) was similar to that of ICa with BAPTA in the pipette. Potentiation of IBa was largely blocked in Ca(2+)-depleted cells when CaCl2 was omitted from the pipette. 7. When ICa was activated at frequencies of > or = 0.1 Hz, with 1 mM CaCl2 added to the nitr-5 (2 mM) in the pipette, the onset of the flash-induced potentiation was best fitted by two exponentials; one was similar to the single component seen at 0.05 Hz and the other was approximately one order of magnitude faster. The contribution of the faster component was positively correlated to the stimulation frequency. 8. The flash-induced potentiation of ICa was suppressed in the presence of a supramaximal concentration of the beta-adrenergic agonist isoprenaline.(ABSTRACT TRUNCATED AT 400 WORDS)