title: Effect of amiloride on diaphragmatic contractility: evidence of a role for Na(+)-Ca2+ exchange.
abstract: Extracellular Ca2+ has been shown to be important for the normal function of the diaphragm. In this study we have examined the potential importance of Na(+)-Ca2+ exchange as a mechanism for Ca2+ influx during the contractile process by studying the effect of inhibition or stimulation of Na(+)-Ca2+ exchange. Blockade of Na(+)-Ca2+ exchange with amiloride attenuated the twitch response, altered the force-frequency response curve, and enhanced the development of fatigue. The effect of amiloride could be partially reversed by increasing the extracellular Ca2+ concentration. The ability of amiloride to decrease force was associated with decreased Ca2+ uptake by the diaphragm. Enhancing intracellular Na(+)-extracellular Ca2+ exchange by inhibiting the Na(+)-K+ pump significantly decreased the rate of the development of muscle fatigue (89%). The maximal inhibition of diaphragmatic force produced by the amiloride analogue benzamil, which possesses 10-fold greater selectivity for Na(+)-Ca2+ exchange, was not significantly different from that produced by amiloride (76.2 +/- 1.1%), with a concentration that decreased maximum force by 50% equal to 46 microM compared with 460 microM for amiloride. Both agents slowed the maximal rate of relaxation up to 90%. Benzamil elevated resting tension during continuous stimulation of the diaphragm at 0.15 Hz. The results suggest that Na(+)-Ca2+ exchange may have a role in the normal function of the diaphragm.
authors.affiliation: Department of Pharmacology, Louisiana State University Medical Center, Shreveport.
authors.forename: J H
authors.initials: JH
authors.lastname: Zavecz
authors.forename: W M
authors.initials: WM
authors.lastname: Anderson
authors.forename: B
authors.initials: B
authors.lastname: Adams
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D003964:Diaphragm / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D007474:Ion Exchange; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*
chemical_list: C013407:benzamil; D000584:Amiloride; D012964:Sodium; D002118:Calcium