Calcium-mediated effects of monovalent ionophores on myocyte contractility and lymphocyte capping: Sodium-modulated calcium activation

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)



First Committee Member

Berton C. Pressman - Committee Chair


Although it is known that monensin, a monovalent carboxylic ionophore, increases Na$\sp+\sb{\rm i}$ and contractility in whole hearts, its mechanisms at a cellular level are not definitively understood. Preliminary experiments involving receptor capping in T-lymphoma cells indicated that monensin has direct effects at the single cell level. Cultured neonatal rat myocytes provided a system of single excitable cells in which the effects of monensin on contractility and Ca$\sp{2+}$ metabolism could be investigated. Through the use of a video-computer system for monitoring contractile activity, and the fluorescent probe fura-2 for quantitation of the Ca$\sp{2+}\sb{\rm i}$ wave, changes in the contractile state of the myocyte were found to correlate with changes in the amplitude of the Ca$\sp{2+}\sb{\rm i}$ wave. In order to distinguish inotropic effects from chronotropic effects, myocytes were pace by field stimulation. Monensin was found to have a significant inotropic effect in myocytes, indicating it is capable of affecting cardiac muscle directly. The inotropic effect appears to be due to indirect enhancement of the Ca$\sp{2+}\sb{\rm i}$ wave, which results from Ca$\sp{2+}$ influx mediated by Na$\sp+$-Ca$\sp{2+}$ exchange. Ionophore-induced influx of Na$\sp+$ was found to be more effective at increasing contractility than inhibition of the Na$\sp+$ pump by ouabain. Monensin and ouabain interacted synergistically to produce greater increases in Na$\sp+\sb{\rm i}$, and Ca$\sp{2+}\sb{\rm i}$, than would have been predicted on a strictly additive basis. Monovalent ionophores are more effective than divalent ionophores at producing useful increases in Ca$\sp{2+}\sb{\rm i}$. Divalent ionophores increase the Ca$\sp{2+}$-permeability of intracellular membranes and disrupt the Ca$\sp{2+}$ storage mechanisms of the sarcoplasmic reticulum. Monensin, ouabain, and veratridine, which alter Na$\sp+\sb{\rm i}$ by different mechanisms, had qualitatively different effects on contractility and the Ca$\sp{2+}\sb{\rm i}$ transient in the cultured neonatal rat myocyte. The agents may induce Na$\sp+$ fluxes of different magnitudes, or compartmentalization may be involved.It is concluded that the monensin-induced increase in Ca$\sp{2+}\sb{\rm i}$ found here in individual cultured myocytes is the cellular basis for direct stimulation of cardiac contractility by the monovalent carboxylic ionophores. In general, increases in intracellular Na$\sp+\sb{\rm i}$ may modulate changes in Ca$\sp{2+}\sb{\rm i}$ that precede stimulation of cardiac contractility in particular, and activation of cellular processes in general.


Biology, Animal Physiology

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