Effects of inhibition of transsarcolemmal calcium influx by nickel on force of postrest contraction and on contracture induced by rapid cooling.

OBJECTIVE

The aim was to characterise the influence of a decrease in transsarcolemmal calcium influx on the calcium content of the sarcoplasmic reticulum and on the force of postrest contraction, which is largely activated by calcium released from the sarcoplasmic reticulum.

METHODS

Isometric force of postrest contractions in isolated rabbit papillary muscles was measured in the absence and in the presence of ryanodine (1 microM). The force measured in the presence of ryanodine was used as an index of transsarcolemmal calcium influx. The calcium content of the sarcoplasmic reticulum was estimated by rapid cooling induced contracture measurements. Nickel (Ni2+) was used to decrease the calcium influx, and the results were compared to those obtained by lowering the extracellular calcium concentration.

RESULTS

The addition of Ni2+ or lowering extracellular Ca2+ markedly decreased the force of postrest contraction measured in the presence of ryanodine, and moderately decreased the force of postrest contraction measured in the absence of ryanodine. Neither the addition of Ni2+ nor lowering extracellular Ca2+ decreased maximum rapid cooling induced contracture; this was actually increased in the presence of Ni2+ even though a larger number of stimuli was required in the presence than in the absence of Ni2+ for the force of the subsequently elicited contracture to reach the maximum level.

CONCLUSIONS

The results suggest: (1) that a decrease in the calcium influx reduces the force of the postrest contraction by decreasing the amount of calcium required to trigger the release of calcium from the sarcoplasmic reticulum, without decreasing its maximum calcium content; and (2) that Ni2+ decreases the rate of filling of sarcoplasmic reticular calcium stores even though the calcium content of the sarcoplasmic reticulum after a sufficiently large number of beats is higher in the presence of Ni2+ than in its absence.