Barium-induced contraction of rat vas deferens in calcium-free solution

Barium-induced contraction of smooth muscle through membrane depolarization is well known. Participation of calcium in Ba2+-induced contraction of smooth muscle has been emphasized. The source of the activator Ca2+ translocated by Ba2+ is, however, equivocal. The present study employing whole rat vas deferens, compared the Ba2+ response of the tissue bathed in Ca2+-deprived salt solution with those in normal salt solution. The results indicate that "phasic" component of Ba2+-induced contraction was due to both mobilization of intracellular calcium and influx of extracellular calcium, which is bound to the membrane while the "tonic" component was entirely due to the influx of the extracellular calcium of the medium. The results also suggest that Ba2+ might induce a contraction independent of Ca2+. This Ca2+-independent Ba2+ contraction is blocked by verapamil. The data has been interpreted to suggest that the calcium channels in rat vas deferens might be of two types--"fast" and "slow" voltage dependent channels--and that fast channels might be more responsible to transmit Ca2+ for phasic component and slow channels for tonic component.