Die beheer van hartspierkontraktiliteit deur die beweging van kalsiumione deur die sarkolemma

Abstract

In this study 45 Ca-fluxes and contractures were done. Characteristics of contractures at low extracellular K+-concentrations and low extracellular Na+-concentrations were evaluated by means of 45 Ca-fluxes. Experimental results, locally done, show an increase in the force of contraction and then a decline at about 40 mM K+. The K+-concentration in the perfusion medium was progressively increased. This is in contrast with results which show a constant value in the force of contraction when a certain K+_ concentration was reached (fig. 1). According to the first hypothesis the efflux of 45Ca is responsible for the characteristics of the relationship between the force of contraction and the membrane potential (extracellular K+-concentration). It was observed in this study that 45Ca-efflux can be responsible for this characteristics. The decline in the force of contraction after a maximum was reached is thus related to 45Ca-efflux. An increase in 45Ca-influx was observed with an increase from 5,4 mM K+ to 25 mf.1 K+ as well as with an increase from 25 mM K+ to 80 mM K+. The increase in 45Ca-influx can explain the initial increase in contractility, but not the decline in contractility at higher K+-concentrations. According to the second hypothesis the increase in mechanical force, with an increase in the extracellular Na+-concentration from Oto 20 mM Na+ (sustained protagonism) can be explained byan increase in 45 Ca-influx.

The contracture results in this study indicate that sustained protagonism can not always be observed when the Na+-concentration is altered from Oto 14 mM Na+. With 4 ta-fluxes and the determination of the total Ca-content {when the muscle was adapted for 1 hour at O mM Na+) the experimental results offer a possible explanation for the sustained protagonism. The corresponding mechanical results however,did not show sustained protagonism. In experiments in which adaptation took place in normal Tyrode, it were shown that sustained protagonism depends on the medium in which the adaptation took place. It is suspected that sustained protagonism depends on 0 mM Na+in the perfusion medium A protagonistic effect with a short duration could be observed in contracture experiments with guinea-pig heart muscle (fig. 12). To get more information about this effect 45 Ca-fluxes should be done in a very short time. In contracture experiments with rabbit heart muscle a reversal was observed after approximately 2 hours. In other words it took a certain time before the protagonistic effect could be observed. The results indicate possible damage of the muscle at 0 mM Na+ when a certain time has elapsed. These results also indicate possible specie differences which may play a role in the inter= pretation of the results.