The effect of cardiac glycosides on the Na+ pump current-voltage relationship of isolated rat and guinea-pig heart cells.
1. Whole-cell recording from isolated rat and guinea-pig ventricular myocytes revealed a change of the cardiac Na+ pump current (Ip)-voltage (V) relationship by cardiac glycosides, specific inhibitors of the Na(+)-K+ pump. 2. Dihydro-ouabain (DHO) diminished Ip in rat ventricular cells at 0 mV in a concentration-dependent manner. 3. The concentration-response curve of Ip inhibition caused by DHO was shifted to higher [DHO] at higher extracellular K+ concentrations ([K+]o) or at more negative membrane potentials. 4. In rat myocytes, DHO immediately flattened the normalized cardiac Ip-V curve and evoked or enhanced a region of negative slope. 5. Ouabain, at concentrations which caused a comparable inhibition of Ip, exerted DHO-like effects on the Ip-V relationship of rat ventricular myocytes. However, the effects developed more slowly. 6. A slowly developing alteration of the Ip-V curve was also observed upon application of DHO to guinea-pig ventricular cells. The range of [DHO] used was about 100-fold lower than that applied to rat ventricular cells, but was equally effective for Ip inhibition. 7. Increasing the K+ concentration of DHO-containing media affected the existing equilibrium of DHO binding to the cardiac Na(+)-K+ pump. A new equilibrium was reached within about 3 s in rat ventricular myocytes, but only within about 50 s in guinea-pig ventricular cells under the experimental conditions chosen. 8. It is concluded that the changes of the cardiac Ip-V curve induced by cardiac glycosides are mediated by voltage-dependent variations of the local [K+]o at the K+ binding sites of the Na(+)-K+ pump in an 'access channel'. The variations were estimated by means of the Boltzmann equation. The estimations agreed with those derived from the measured DHO binding to the Na(+)-K+ pump at various [K+]o. A new equilibrium of glycoside binding to the pump is established at the altered [K+]o. The time necessary to reach the new binding equilibrium varies with the cardioactive steroid, its concentration and the glycoside sensitivity of the cardiac cells.
Hermans
A N
AN
Interdisciplinary Research Centre, Catholic University of Leuven, Kortrijk, Belgium.
Glitsch
H G
HG
Verdonck
F
F
D000818:Animals; D002301:Cardiac Glycosides / Q000494:pharmacology*; D004594:Electrophysiology; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects* / Q000294:innervation; D006352:Heart Ventricles / Q000166:cytology / Q000294:innervation; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D010042:Ouabain / Q000031:analogs & derivatives / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000502:physiology; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects*; D016276:Ventricular Function
D002301:Cardiac Glycosides; C000977:dihydroouabain; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium