The relationship between contraction and intracellular sodium in rat and guinea‐pig ventricular myocytes.

Abstract

SUMMARY1. The contraction, measured optically, and the intracellular Na' activity (aNa), measured with the Na+-sensitive fluorescent dye SBFI, have been recorded simultaneously in rat and guinea-pig ventricular myocytes.2. In rat and guinea-pig ventricular myocytes at rest, a'a was 7 8 + 03 mM (n = 4) and 5-1 +0-3 mM (n = 16), respectively.3. When both rat and guinea-pig ventricular myocytes were stimulated at 1 Hz after a rest there was usually a gradual increase in twitch shortening (referred to as a 'staircase') over several minutes accompanied by an increase in a'a over a similar time course. Twitch shortening increased by 21+3 % (n = 6) and 20 + 4% (n = 16) (of steady-state twitch shortening during 1 Hz stimulation) per millimolar rise in aNa in rat and guinea-pig ventricular myocytes, respectively.4. When rat and guinea-pig ventricular myocytes were exposed to strophanthidin to block the Na+-K' pump, there were increases in twitch shortening and aia over similar time courses. Twitch shortening increased by 24 + 4 % (n = 5) and 20+3 % (n = 10) (of control twitch shortening) per millimolar rise in aNa in rat and guineapig ventricular myocytes respectively. 5. The inotropic effect of cardiac glycosides, such as strophanthidin, is widely regarded to be principally the result of the rise in aia. The similarity of the relation between twitch shortening and aia during the staircase and on application of strophanthidin suggests that the progressive increase in the strength of contraction during the staircase was also linked to the rise in aNa.6. In guinea-pig, but not rat, ventricular myocytes there was hysteresis in the relation between twitch shortening and a'a on application and wash-off of strophanthidin. This indicates that strophanthidin has another inotropic action in guinea-pig ventricular myocytes.7. A computer model of excitation-contraction coupling has been developed to simulate the staircase and the action of cardiac glycoside and to account for the relation between contraction and intracellular Na+.

The role of the Na(+)‐Ca2+ exchanger in the rate‐dependent increase in contraction in guinea‐pig ventricular myocytes.

“…The cells were allowed to settle for several minutes onto the glass bottom of the chamber before being superfused at a rate of 3 ml min-' with a physiological salt solution of the following composition (mM): NaCl, 140; KCl, 5-4; MgCl2, 1-2; NaH2PO4, 0 4; Hepes, 5 0; glucose, 10 0; CaCl2, 18; pH 7*4 at 36 'C. (Harrison et al 1992b). Cell length was recorded using an optical system based on a photodiode array (Boyett, Moore, Jewell, Montgomery, Kirby & Orchard, 1988) and displayed on a chart recorder.…”

“…In some cases, cells were loaded with the acetoxymethyl ester of SBFI (SBFI AM; see Harrison et al 1992b). Fifty micrograms of SBFI AM were dissolved in 20 #sl of dimethyl sulphoxide (DMSO) and 4 pl1 of 20% Pluronic F127 (Molecular Probes, USA; in DMSO); 12 #1 of this SBFI stock was then added to 2 ml of cells ([SBFI] of 11 uM) and the cell suspension gently agitated for 2 h (Moore, Minta, Tsien & Fay, 1989;Harrison et al 1992b). The cells were then centrifuged and the pellet resuspended in the physiological salt solution containing 500 /M CaCl2 and stored in this solution until required.…”

“…Ventricular myocytes were prepared using a standard collagenase/protease dispersion technique that has been described previously in full (Harrison et al 1992b). In some cases, cells were loaded with the acetoxymethyl ester of SBFI (SBFI AM; see Harrison et al 1992b).…”

“…Ventricular myocytes were prepared using a standard collagenase/protease dispersion technique that has been described previously in full (Harrison et al 1992b). In some cases, cells were loaded with the acetoxymethyl ester of SBFI (SBFI AM; see Harrison et al 1992b). Fifty micrograms of SBFI AM were dissolved in 20 #sl of dimethyl sulphoxide (DMSO) and 4 pl1 of 20% Pluronic F127 (Molecular Probes, USA; in DMSO); 12 #1 of this SBFI stock was then added to 2 ml of cells ([SBFI] of 11 uM) and the cell suspension gently agitated for 2 h (Moore, Minta, Tsien & Fay, 1989;Harrison et al 1992b).…”

Mechanical Restitution in Atrial Muscle from Human and Rat Hearts: Effects of Agents that Modify Sarcoplasmic Reticulum Function

“…Hence post-rest decay is prominent in situations favouring the Ca2+ efflux mode of the Na+/Ca2+ exchanger during rest, whereas potentiation is induced when the exchanger operates in the Ca2+ influx mode. The high level of intracellular Naf concentration in rat myocardium (Harrison et al 1992) would indeed promote Ca2+ entry via Na'i Ca'+ exchange during rest leading to prominent post-rest potentiation (Benijamali et al 1991), whereas human atrial myocardium would resemble rabbit in its low intracellular Na+ concentration which favours Ca2+ removal from the cell and hence results in post-rest decay (Bassani & Bers 1994).…”