Effects of aminoglycoside antibiotics on bound calcium and contraction in guinea-pig atria.

We studied the effect of three aminoglycoside antibiotics which have been shown to replace Ca from lipid monolayers on the superficially bound Ca of isolated beating left atria from guinea-pigs. The cellular Ca content was determined by means of 45Ca after having attained complete exchange. The antibiotics dibekacin, sisomicin, and gentamicin, all reduced the cellular Ca content by 10-20% in a dose-dependent manner. The loss of superficially bound Ca was accompanied by a decline of the contractile force by 40-90%. It is concluded that in isolated atrial muscle it is the amount of Ca bound to the outer surface of the cardiac plasmalemma, rather than the extracellular Ca2+ concentration, that determines contraction height.