Title

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

Date of Award

1988

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Pharmacology

First Committee Member

Berton C. Pressman, Committee Chair

Abstract

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.

Keywords

Biology, Animal Physiology

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:8910903