The effect of external sodium on the contracture responses of pacemaker preparations of Leptodactylus insularis
Morales, P.; Caputo, C.; Bolaños, P.; López, J.R.
Acta Cientifica Venezolana 37(1): 43-50
1986
ISSN/ISBN: 0001-5504 PMID: 3825432 Document Number: 280376
The effects of external sodium concentration on the contracture responses of atrioventricular pacemaker preparation, dissected from the tropical toad L. insularis, have been studied. When exposed to sodium-free solutions, the pacemaker strips develop a slowly raising response which relaxes spontaneously, and whose peak tension is larger, when the preparation is depolarized. This response is not obtained with external sodium concentrations greater than 20 mM. In the presence of external sodium, raising the external potassium concentration, causes the development of a fast, twitch-like response which relaxes rapidly; in the absence of external sodium, this first response is followed by a slower tension component, resembling that obtained in the sodium-free solutions alone, but whose peak tension is dependent on both the external sodium and potassium concentrations. When the external potassium is 100 mM, a tonic tension component can be developed even when the external sodium concentration is as high as 80 mM. The relaxation rates of both the Na-free and the high K, Nafree responses are proportionally related to the external sodium concentration. These relaxation are also affected by the membrane potential, being slower when the preparations are depolarized. When caffeine, even at large concentrations (up to 50 mM) is added to the normal Ringer's solution, only a fast, initial potentiation of the spontaneously generated twitch responses, which may later disappear, can be observed. On the other hand, caffeine contractures can be developed, even at low drug concentration (1.2 mM), following the spontaneous relaxation of responses induced in sodium-free media. This caffeine rsponse can be abolished, when the external sodium is increased, but not when this is done in the presence of 3 mM Lanthanum. The results obtained in this work support the idea that Na-Ca exchange participates in the regulation of cardiac contractility, operating in it's two modes: forward mode, by extruding calcium and favoring relaxation, and reverse mode, by contributing to calcium entry, and to contractile activation. In both modes the exchange appears to be dependent on the membrane potential.