Sarcoplasmic reticulum and Na+/Ca2+ exchanger function during early and late relaxation in ventricular myocytes

• Published in: American journal of physiology. Heart and circulatory physiology 1997-12, Vol.273 (6), p.H2765
• Main Authors: Yao, Atsushi, Matsui, Hiroshi, Spitzer, Kenneth W, Bridge, John H. B, Barry, William H
• Format: Article
• Language: English
• ISSN: 0363-6135; 1522-1539
• DOI: 10.1152/ajpheart.1997.273.6.h2765

The Division of Cardiology and the Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah Health Sciences Center, Salt Lake City, Utah 84132 The relative importance of the Na + /Ca 2+ exchanger in the initial and terminal phases of relaxation and the decline in the [Ca 2+ ] i transient was investigated in adult rabbit ventricular myocytes loaded with the Ca 2+ indicator fluo 3. For electrically stimulated contractions, the peak intracellular Ca 2+ concentration ([Ca 2+ ] i ) was 700 ± 87 nM and end-diastolic [Ca 2+ ] i was 239 ± 30 nM (0.25 Hz, 37°C, 1.08 mM extracellular Ca 2+ concentration; n  = 14). Abrupt inhibition of Na + /Ca 2+ exchange was produced by removal of extracellular Na + (KCl substitution) and Ca 2+ [2 mM Ca 2+ -free ethylene glycol-bis( -aminoethyl ether)- N , N , N ', N '-tetraacetic acid] by means of a rapid switcher device (SW). Abrupt exposure to high K + induced an action potential, although sufficient Ca 2+ remained adjacent to the sarcolemma to induce a contraction (SW beat) and [Ca 2+ ] i transient that were identical in amplitude to those induced by electrical stimulation (ES beat). The initial relaxation and decline in the [Ca 2+ ] i transient was not significantly prolonged by abrupt elimination of the Na + /Ca 2+ exchanger, but the rate and extent of the terminal phase of the decline in the [Ca 2+ ] i transient were significantly reduced. The first derivative of [Ca 2+ ] i with respect to time versus [Ca 2+ ] i during the decline of the [Ca 2+ ] i transient attributable to sarcoplasmic reticulum (SR) function was estimated from the average SW transients, and that attributable to Na + /Ca 2+ exchange was estimated from the difference between SW and ES transients. By this analysis, the Na + /Ca 2+ exchanger produces 13% of the first half of the decline in [Ca 2+ ] i and 45% of the second half of the decline. We conclude that abrupt inhibition of forward Na + /Ca 2+ exchange does not significantly affect the amplitude or the initial rate of decline of the [Ca 2+ ] i transient and relaxation. However, its contribution to the reduction of [Ca 2+ ] i becomes apparent late during the [Ca 2+ ] i transient, when cytosolic [Ca 2+ ] i has been reduced. sodium/calcium exchange; fluo 3; relaxation; calcium ion transient