J
Mol Cell Cardiol
24, 967-976
(1992)
The Effects of Blocking the Na-Ca Exchange the Physiological Contraction-Relaxation Myocyte B. Lewartowski,
B. M. Wolska
and
at Intervals Throughout Cycle of Single Cardiac
K. Zdanowski
Department of Clinical Physiology, Medical Center of Postgraduate Education, PL 01-813, Warsaw, Poland (Received 18 Jub
1991; accepted in revisedform
18 March,
Mal-ymoncka 99.
1992)
B. LEWARTOWSKI, B. M. WOLSKA AND K. ZDANOWSKI. The Effects of Blocking the Na-Ca Exchange at lntrrvals Throughout the Physiological Contraction-Relaxation Cycle of Single Cardiac Myocyte. Journal of Molecular and Gllular Cardiology (1992) 24, 967-976. The method of rapid superfusion of the single isolated ventricular myocytes of guinea-pig heart was used in order to inhibit the Na-Ca exchange throughout the physiological contraction-relaxation cycle. Superfusion of the cell at selected intervals during the contraction with the Na.Cafree solution resulted in increase in its amplitude, increase in time to peak shortening and in delay of relaxation, albeit the cells relaxed before reperfusion of normal Tyrode solution. The largest increase in amplitude of contraction (to 134f 16%) was observed when the effective exchange of the cell’s environment was attained h 50 ms after the pulse stimulating contraction. The effects declined promptly when the delay was increased beyond 100 ms. In the cells treated with 10 rnM caffeine superfusion with the Na,Ca-free solution after thr drla! of 50-100 ms resulted in decrease in extent of shortening. Increase in delay resulted in slight increase in rxtent of shortening with respect to control and strong inhibition of relaxation. The strongest effects were observed when the delay was w 200 ms. Superfusion of the normal cells and of the cells treated with caffeine between contractions resulted in slight potentiation of the next beat. It is concluded that Na-Ca exchange provides an important mechanism of relaxation and outward Ca’+ transport in thr physiological contraction of thv ventricular cardiomyocyte. KEY
\~ORDS:
Na-Ca
exchange;
Cardiac
myocyte;
Contraction-relaxation
Introduction Under the steady state stimulation conditions Ca’+ entering cardiac myocyte during the initial parts of the plateau of an action potential (AP) must be extruded from the cell before the next excitation. Indeed, Hilgemann (1986b) has shown that there is a net Ca” uptake by the cell over the initial parts of an AP and the net CazC loss over its later parts. It is generally believed that the Na-Ca exchange provides the main route of the outward Ca2+ transport (Hilgemann and Noble, 1987; Philipson, 1990; Powell and Noble, 1989; Wier, 1990). The exchange has been shown to be very effective in removing the Ca2+ released from sarcoplasmic reticulum (SR) by caffeine from the cell (Callevaert et al., 1989) or Ca*+ which has entered vnlPlease address all correspondence to: Department Marvmoncka 99. PL 01-813, Warsaw, Poland.
0022%2828/92/090967 + 10 $08.00/O
of Clinical
cycle.
tage-clamped myocyte, treated with caffeine through the activated Ca’+ channels (Bridge et al., 1990). Thus the Na-Ca exchange working in the “Ca’+ efflux mode” provides the important mechanism by which the sarcoplasmic Ca*+ concentration elevated during excitation may be brought to the pre-excitation level. Another important mechanism is provided by the reuptake of Ca’+ by the SR. The relative quantitative contribution of these mechanisms to relaxation is of considerable interest. This problem has been addressed by Bers and Bridge (1989) who found that relaxation of ventricular muscle upon rewarming after cooling contracture is little affected by removal of Na, and Ca
Mol Cell Cardiol
24, 967-976
(1992)
The Effects of Blocking the Na-Ca Exchange the Physiological Contraction-Relaxation Myocyte B. Lewartowski,
B. M. Wolska
and
at Intervals Throughout Cycle of Single Cardiac
K. Zdanowski
Department of Clinical Physiology, Medical Center of Postgraduate Education, PL 01-813, Warsaw, Poland (Received 18 Jub
1991; accepted in revisedform
18 March,
Mal-ymoncka 99.
1992)
B. LEWARTOWSKI, B. M. WOLSKA AND K. ZDANOWSKI. The Effects of Blocking the Na-Ca Exchange at lntrrvals Throughout the Physiological Contraction-Relaxation Cycle of Single Cardiac Myocyte. Journal of Molecular and Gllular Cardiology (1992) 24, 967-976. The method of rapid superfusion of the single isolated ventricular myocytes of guinea-pig heart was used in order to inhibit the Na-Ca exchange throughout the physiological contraction-relaxation cycle. Superfusion of the cell at selected intervals during the contraction with the Na.Cafree solution resulted in increase in its amplitude, increase in time to peak shortening and in delay of relaxation, albeit the cells relaxed before reperfusion of normal Tyrode solution. The largest increase in amplitude of contraction (to 134f 16%) was observed when the effective exchange of the cell’s environment was attained h 50 ms after the pulse stimulating contraction. The effects declined promptly when the delay was increased beyond 100 ms. In the cells treated with 10 rnM caffeine superfusion with the Na,Ca-free solution after thr drla! of 50-100 ms resulted in decrease in extent of shortening. Increase in delay resulted in slight increase in rxtent of shortening with respect to control and strong inhibition of relaxation. The strongest effects were observed when the delay was w 200 ms. Superfusion of the normal cells and of the cells treated with caffeine between contractions resulted in slight potentiation of the next beat. It is concluded that Na-Ca exchange provides an important mechanism of relaxation and outward Ca’+ transport in thr physiological contraction of thv ventricular cardiomyocyte. KEY
\~ORDS:
Na-Ca
exchange;
Cardiac
myocyte;
Contraction-relaxation
Introduction Under the steady state stimulation conditions Ca’+ entering cardiac myocyte during the initial parts of the plateau of an action potential (AP) must be extruded from the cell before the next excitation. Indeed, Hilgemann (1986b) has shown that there is a net Ca” uptake by the cell over the initial parts of an AP and the net CazC loss over its later parts. It is generally believed that the Na-Ca exchange provides the main route of the outward Ca2+ transport (Hilgemann and Noble, 1987; Philipson, 1990; Powell and Noble, 1989; Wier, 1990). The exchange has been shown to be very effective in removing the Ca2+ released from sarcoplasmic reticulum (SR) by caffeine from the cell (Callevaert et al., 1989) or Ca*+ which has entered vnlPlease address all correspondence to: Department Marvmoncka 99. PL 01-813, Warsaw, Poland.
0022%2828/92/090967 + 10 $08.00/O
of Clinical
cycle.
tage-clamped myocyte, treated with caffeine through the activated Ca’+ channels (Bridge et al., 1990). Thus the Na-Ca exchange working in the “Ca’+ efflux mode” provides the important mechanism by which the sarcoplasmic Ca*+ concentration elevated during excitation may be brought to the pre-excitation level. Another important mechanism is provided by the reuptake of Ca’+ by the SR. The relative quantitative contribution of these mechanisms to relaxation is of considerable interest. This problem has been addressed by Bers and Bridge (1989) who found that relaxation of ventricular muscle upon rewarming after cooling contracture is little affected by removal of Na, and Ca
