EFFECT OF DIVALENT-CATIONS ON THE CONTRACTILE RESPONSE OF RAT AORTA TO DEPOLARIZATION BEFORE AND AFTER NIFEDIPINE TREATMENT

The influence of the divalent cations, Ca2+, Mg2+ and Ba2+, on the con tractile response of the rat aorta to KCI and on the recovery of this response after nifedipine treatment was analyzed. KCl (80 mmol/l) prom oted a two-phase (phasic and tonic) contractile response in Krebs solu tion but, as expected, no contractile response in Ca2+-free medium. In Mg2+-free medium, the phasic response to KCI was unaffected but the t onic cone decreased slowly, suggesting that a long incubation time in the absence of Mg2+ (65 min) promotes a loss of or a change in the int racellular distribution of this ion that modifies Ca2+ entry through L channels or Ca2+ handling. Ba2+ (1.8 mmol/l) contracted the rat aorta in the absence or presence of Ca2+ but, when Ca2+ was not present, Ba 2+ modified the contractile process so that a new addition of KCI did not reproduce the contractile response. After nifedipine treatment, no phasic response to KCI was observed and only a slow response appeared that increased as long as the KCl was present (45 min). After washing , KCl induced a phasic response similar to the standard response, but a smaller tonic one. Divalent cations accelerated However, the recover y of the Ca2+ channels related to the phasic contraction was independe nt of divalent cations. the recovery of the channels involved in the t onic contraction.