Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart.
1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.
Volkmann
R
R
Department of Clinical Physiology, University of Göteborg, Sweden.
D000818:Animals; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D007474:Ion Exchange; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D011898:Ranidae; D012508:Sarcolemma / Q000378:metabolism; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology
D012964:Sodium; D002118:Calcium