A 40-minute perfusion of the frog ventricular strip with a hypoxic Ringer solution brings about a decline in action potential duration by 37 plus or minus 10% whereas the contractility is completely suppressed by that time. The time constant of the contractility exponential decline constitutes 8 plus or minus 2 min. the effect of hypoxia is reversible. Complete blockade of the calcium channels by fenigidin (3.5 x 10(-5) M) irreversibly lowers action potential duration by 62 plus or minus 2%. Contraction is completely suppressed on attaining the given level of action potentials. Hypoxia does not virtually affect action potential after the preliminary blockade of the calcium channels by fenigidin. It is suggested that the hypoxic solution mainly blocks the inward current across the calcium channels without affecting the potassium permeability of the sarcolemma. Meanwhile complete suppression of the contractility under hypoxia cannot be accounted for only by the blockade of the calcium channels.
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