The voltage dependence of contraction at different stimulation rates in guinea‐pig ventricular myocytes

Abstract

SUMMARYVentricular myocytes, isolated from the guinea-pig, were stimulated to contract by 100 ms long voltage clamp pulses from -80 to 0 mV at 0-5 and 3 Hz. An increase in frequency from 0-5 to 3 Hz led to a positive inotropic effect. Contraction-voltage relationships (CVR) were determined at each frequency. The CVR at 0.5 Hz was bell shaped and peaked between 0 and +20 mV, displaying a voltage dependence similar to the L-type Ca2+ current (ICa). At 3 Hz, contractions continued to increase at positive voltages, giving a more sigmoidal CVR. At 0-5 Hz, TTX reduced the size of steady-state contractions to 91 + 2 % of control values, but had no effect on the shape of the CVR. At 3 Hz, TTX significantly reduced (P < 0.05) the magnitude of contractions at positive voltages (> +20 mV) but had no significant effect on contractions at voltages negative to 0 mV. These data illustrate that intracellular sodium activity (a' a) and, in particular, Na+ entry due to the sodium current (INaJ are important in determining the voltage dependence of contraction at positive voltages. Thapsigargin (2-5 ,uM), a blocker of the sarcoplasmic reticulum (SR) Ca2+-ATPase, reduced the size of steady-state contractions at 0 mV to 65 ± 7 % at 0-5 Hz. Increasing frequency to 3 Hz abolished the positive inotropy seen under control conditions. With thapsigargin present, contractions at 0-5 Hz were reduced at all potentials and the CVR was bell shaped. At 3 Hz the CVR was sigmoidal in shape. Contractions were significantly inhibited by thapsigargin at all potentials, but most significantly at more positive potentials ( > +20 mV). These data show that, at normal body temperature, the shape of the CVR of guinea-pig ventricular myocytes changes with stimulation rate. Due to the voltage dependence of ICa, contractions evoked at positive voltages at 3 Hz must be supported by other mechanisms. The sensitivity of such contractions to TTX and thapsigargin suggests the involvement of both a Na+-dependent process and the SR. One possibility is that when a'a and the Ca2+ content of the SR are raised at higher stimulation rates, enhanced Ca2+ entry via reverse Na+-Ca2+ exchange leads to a direct activation of the myofilaments and, to a lesser extent, the release of Ca2+ from the SR.

Cellular and subcellular alternans in the canine left ventricle

“…Immunocytochemistry displayed RyRs reflecting sarcomeric organization but no organized t-tubules. Available evidence suggests that E-C coupling occurs as a result of direct myofilament activation by Ca 2ϩ entering via L-type Ca 2ϩ channels as well as via Na ϩ /Ca 2ϩ exchange (14). The fact that Ca 2ϩ alternans does not occur under these conditions supports the hypothesis that SR Ca 2ϩ release and normal SR function are critical for the development of alternans.…”

Quantitative analysis of Na+‐Ca2+ exchanger expression in guinea‐pig heart

“…Under normal circumstances, NCX acts predominately as a Ca 2+ extrusion system but under certain circumstances (e.g. as intracellular Na + activity increases during an increase in stimulation rate [15]), the prospect of Ca 2+ entry via reverse mode NCX is increased (and the driving force for Ca 2+ extrusion is reduced) which leads to an increase in the Ca 2+ content of the sarcoplasmic reticulum (SR) and a consequent increase in the strength of cardiac contractions. Furthermore, it has been demonstrated in ventricular cells [16,17] that Ca 2+ entry via reverse mode Na + ‐Ca 2+ exchange could, under certain circumstances, evoke the release of Ca 2+ from the SR [18].…”