DEPOLARIZATION-INDUCED CA ENTRY VIA NA-CA EXCHANGE TRIGGERS SR RELEASE IN GUINEA-PIG CARDIAC MYOCYTES

In mammalian heart muscle, Ca entry through L-type Ca channels is thou ght to be the primary trigger for the sarcoplasmic reticulum (SR) Ca r elease, which initiates contraction. The results of this study show th at, in guinea pig myocytes with a normal internal Na (10 mM Na in pipe tte), another trigger mechanism for SR release and contraction exists. A crucial feature of these experiments was the ability to change rapi dly the extracellular environment of a single myocyte so that alterati ons of intracellular Ca and SR Ca load were minimized for each solutio n change. We found the following results. 1) A switch to Na-free solut ion 50 ms before depolarization led to an increase of phasic contracti on without increasing L-type Ca current I-Ca or Ca loading of the SR. 2) Although rapid application of 20 mu M nifedipine 3 s before a + 10- mV pulse blocked I-Ca completely, 43 +/- 11 (SE) % of the phasic contr action remained. Similar results were obtained by rapid switching to 1 50 mu M Cd to block I-Ca. 3) Phasic contraction and I-Ca had different voltage dependence. With steps to positive potentials there was littl e I-Ca but still a substantial phasic contraction. 4) Under action pot ential conditions, 64.6 +/- 7.9% of the control phasic contraction rem ained after switching to 20 mu M nifedipine to block I-Ca. 5) The cont raction remaining with nifedipine was unaffected by adding 100 mu M Ni . Because 100 mu M Ni blocks T-type Ca channels, this shows that Ca en try via T-type Ca channels is not involved in triggering SR release. 6 ) The phasic contraction remaining after a rapid switch to nifedipine was blocked completely by adding 5 mM Ni. Because this concentration o f Ni is known to block the Na-Ca exchange, this result suggests that t he exchange plays a role in triggering SR release. Taken together, the present results indicate that depolarization-induced Ca entry on the Na-Ca exchange is able to trigger SR release and phasic contraction. T his explanation can account for increased phasic contraction after a r apid switch to Na-free solution, persistence of a phasic contraction i n the complete absence of I-Ca, substantial phasic contraction at posi tive test potentials where there is no I-Ca, and abolition of nifedipi ne-resistant contraction by 5 mM Ni.