CALCIUM-INDUCED CONTRACTURE STIMULATES NA,K PUMP RATE IN ISOLATED SHEEP CARDIAC PURKINJE-FIBERS

By keeping intracellular Na+ (a(Na)(i)) low, the Na,K-pump can prevent Ca2+ overload of cardiomyocytes. We therefore examined whether Ca2+ s timulates Na,K-pump activity in sheep cardiac Purkinje fibers. By remo ving Ca2+, Mg2+ and K+, the fibers depolarized and a(Na)(i) rose to 70 mM. After addition of 6 mM Mg2+ and lowering extracellular Na2+ to 29 mM, 3 mM Rb+ was added, and over 10-15 min a(Na)(i) recovered to 3-7 mM. Two load-recovery cycles were conducted in 10 fibers. During one o f the cycles Ca2+ (0.1-1.0 mM) was added before Rb+, causing a contrac ture, During recovery a(Na)(i) fell faster during Ca2+ contracture tha n in control cycles. Between 30 and 20 mM the rates were -10.0+/-1.6 a nd -5.4+/-0.6 mM/min, respectively (P<0.05). In Ca2+-exposed fibers te nsion fell almost parallel with a(Na)(i). Na,K-pump reactivation cause d membrane potential (V-m) to hyperpolarize transiently to -70 mV. Ca2 + did not affect membrane conductance. For a given a(Na)(i) during rea ctivation, V-m was more negative during Ca2+ contracture and depolariz ed faster (P<0.05), Intracellular pH (pH(i)) fell from 7.11+/-0.05 to 6.92+/-0.08 (N.S.) during control load-recovery cycles and was 6.83+/- 0.14 at the end of the Ca2+ cycles, ATP content of the fibers did not change significantly through two complete load-recovery cycles, but cr eatine phosphate (CrP) fell by about 40%. By fitting the data to a mod el incorporating the Hill equation we show that during Ca2+-induced co ntracture maximum Na,K-pump rate (V-max) was increased by about 40% an d a(Na)(i) that causes 50% pump activation (k0.5) was lowered from 21. 2+/-1.6 to 15.5+/-1.4 mM. (C) 1997 Academic Press Limited.