Sodium Requirement for the Positive Inotropic Action of Isoproterenol on Guinea Pig Atria

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Abstract
Isoproterenol does not elicit its characteristic positive inotropic action in contracting guinea pig atria suspended in sodium-free media. However, the ability of isoproterenol to decrease the time to peak tension development during an individual contraction cycle is still present in sodium-free solutions. Removal of sodium diminished but did not eliminate the tissues' ability to elevate adenosine 3′,5′-monophosphate in response to isoproterenol. The striking absence of an inotropic action by isoproterenol on atria in sodium-free media suggests that sodium (and possibly a sodium-calcium exchange across the sarcolemma) plays an important role in the inotropic action of catecholamines.