Possible role of Na-Ca exchange mechanism in the contractions induced in guinea-pig aorta by potassium free solution and ouabain.

Abstract

1. In guinea-pig aorta, K free solution and ouabain (2×10−5 M) produced sustained contractions with similar time course and magnitude. They were only partially inhibited by phentolamine (10−6 M). Experiments were conducted to investigate the mechanisms of these contractions. 2. When K was replenished after tension had reached a maximum in K free solution, the muscle strip slowly relaxed. Rb and Cs ions also diminished the contractions in K free solution and the order of effectiveness was K>Rb>Cs. The relaxation induced by these monovalent cations was inhibited by ouabain (2×10−5 M). 3. Contractions induced by K free solution and ouabain depended on the ratio [Ca]o/[Na]o/2; as long as the ratio [Ca]o/[Na]o/2was kept constant, the magnitude of the contractions did not change even if [Ca]o was lowered. 4. In low Na medium (11.9 mM), K depletion was ineffective and ouabain produced only a small contraction. 5. In K free solution or in the presence of ouabain, the muscle strips gained Na and lost K. The contraction started to develop when [Na]i was 70 mM/kg cell in K free solution or 50 mM/kg cell in 2×10−5 M ouabainsolution and reached its maximum when [Na]i was 85 mM/kg cell in both solutions. 6. The results suggest that these contractions are produced through a Na−Ca exchange mechanism following the accumulation of Na either by K free solution or by ouabain. Furthermore, ouabain seems to have an additive effect on the vascular tone.