Abstract
The transmembrane potential and the isometric contraction were recorded from rabbit left atria driven electrically at 30 and 60/min. Removal of Ca++ from bathing media decreased the resting potential and the overshoot, prolonged the 10% duration and diminished the force of contractions. The addition of Mg++ (2.2 mM) shortened the action potential duration and did not restore contractions. Strontium ions (2.2 mM) when applied to Ca++-free media increased the resting potential and the overshoot, prolonged the action potential duration and augmented the contractility. Norepinephrine (5 x 10-7 to 10-5 M) elicited an increase in the force of contractions but did not significantly influence parameters measured from atrial action potentials. However, at zero [Ca++]o norepinephrine markedly increased the magnitude and the duration of action potentials without modifying the resting potential. Under this condition norepinephrine restored contractions. Caffeine (up to 5 mM) did not restore the magnitude of action potentials and the contraction. The membrane effects and the inotropic effect of norepinephrine were inhibited by the addition of Mg++ to Ca++-free media. The inhibitory effect of Mg++ was more apparent when the preparation was driven at 30/min than at 60/min. In solutions in which Ca++ was substituted with Sr++ action potential durations were prolonged by the amine. The contractile force at driving rates higher than 60/min was reduced by substitution of Sr++ for Ca++. Norepinephrine moved the force-rate curve upward in a dose-dependent manner.
Footnotes
- Received August 2, 1971.
- Accepted November 22, 1971.
- © 1972 by The Williams & Wilkins Co.
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