Electrogenic sodium‐calcium exchange in cultured embryonic chick heart cells.

Abstract

SUMMARY1. The membrane potential (Em) of cultured chick embryonic heart cells depolarized to -36 mV after inhibition of the Na+-K+ pump by 0-1 mM-ouabain in a [K+]. of24 mM: this was accompanied by a rise in Na+ content of -65 % in 3 min. Lowering [Na+]. to 27 mm then caused a fall in Na+ content, a rise in Ca2+ content and a small hyperpolarization of -5 mV. The fall in Na+ content indicated a movement of Na+ which was in the opposite direction to the Na+ electrochemical gradient (a countergradient movement).2. In the presence of 10 mM-Cs+ or 1 mM-Ba2+ the hyperpolarization was -10 or 30 mV, respectively. A 30 mV hyperpolarization took Em negative to the reversal potentials for K+, Na+ and Cl-as measured by ion-selective micro-electrodes.3. The decay of the intracellular Na+ activity, aka, in an [Na+]o of 27 mM followed a simple exponential time course (time constant, 36 s). The initial rate depended on the value to which [Na+]. was lowered in a manner suggesting a simple competitive inhibition of the exchange by external Na+.4. The low- [Na+]. hyperpolarization was unaffected by amiloride (0-1 or 1 mM) or verapamil (20 /SM). Both La3+ (1 mM) and Mn2+ (20 mM) blocked the hyperpolarization sufficiently to prevent Em hyperpolarizing negative to the reversal potentials for K+, Na+ and Cl-. 6. We conclude that cultured embryonic chick heart cells contain a Na+{:Ca2+ exchange evidenced by the ability to cause movements of Na+ and Ca2+ which are counter to their respective electrochemical gradient and which are accompanied by downhill movements of the counter ion. In the presence of Ba2+ (which presumably blocks K+ permeability) the exchange is accompanied by a hyperpolarization which

Effects of sodium‐potassium pump inhibition and low sodium on membrane potential in cultured embryonic chick heart cells.

“…These cultures exhibit movements of Na+ and Ca2+ which can be shown to correlate very closely with those measured on polystrands (Jacob et al 1987). Fig.…”

“…are at least in part due to uncoupled electrodiffusive ion movements so that we cannot conclude that Na+-Ca2+ exchange is electrogenic. In the following article (Jacob, Lieberman & Liu, 1987) we show that under slightly different conditions it is possible to elicit a hyperpolarization that is unequivocally due to an electrogenic Na+-Ca2+ exchange.…”

Ionic Currents and Spontaneous Firing in Neurons Isolated from the Cerebellar Nuclei

“…Potassium channels, including M-channels, inward rectifiers, and TASK channels, tend to bring resting potentials toward E K (Marrion, 1997;Reimann and Ashcroft, 1999;Millar et al, 2000); in several cell types the Na/K pump also serves to hyperpolarize resting potentials (Jacob et al, 1987;Jones, 1989;Trotier and Døving, 1996). Many neurons rest positive to E K , however, indicating that inward currents also must be active at rest (Forsythe and Redman, 1988;Jones, 1989).…”

“…These can include I h (Jones, 1989;Spruston and Johnston, 1992;Lamas, 1998;Doan and Kunze, 1999) and possibly other TTX-insensitive sodium fluxes (Forsythe and Redman, 1988). Additionally, the Na/Ca exchanger has been reported to depolarize resting potentials (Jacob et al, 1987).…”

Relationship between contraction strength and stimulation frequency in cultured chick embryonic heart cells.

“…Probably, SR can accumulate much Ca2 + during prolonged rested period and release much Ca2 + in the following contractions, if the stimulation frequency was low. The Na-Ca exchange mechanism may participate in the strength-frequency relationship, because it is reported that Na-Ca exchange develops well in chicken embryonic cells (JACOB et al, 1987). During long rested period, [Ca2 +]; will be low and Na-Ca exchange is facilitated, which will bring about the potentiation of the first contraction.…”