Effects of sodium and calcium concentration on the barium chloride-induced electrical and contractile responses of the guinea pig vas deferens.
1. The mechanism of 10 mM barium chloride-induced electrical and contractile responses of the guinea pig vas deferens was investigated using calcium channel blockers and by modifying the Na+ and Ca2+ concentrations of the nutritional solution. Isometric contraction and membrane depolarization were measured simultaneously by the sucrose-gap technique. 2. In the absence of added Ca2+ there was a decay of the contractile but not of the depolarizing effect, which was independent of the frequency of 10 mM barium chloride administration. However, both barium-induced contraction and depolarization were reduced and the former more rapidly in a Ca2+-free solution (0.5 mM EGTA). 3. In low-Na+ solution (16 mM) there was a 3-fold increase in depolarization and no change in contraction, but in a low-Na+ Ca2+-free solution (0.5 mM EGTA), only contraction was reduced. 4. Nifedipine blocked both barium-induced contraction and depolarization in a dose-dependent manner when the organs were bathed in regular nutrient solution. In contrast, in low-Na+ solution only 50% of the depolarization was blocked by 10(-6) M nifedipine, which completely inhibited contraction. Both barium-induced contraction and depolarization in low-Na+ Ca2+-free solution were blocked by 10(-3) M LaCl3. 5. These data indicate that in the presence of calcium, barium mobilizes extracellular calcium through voltage-dependent channels and the contraction is due to the entry of calcium. In contrast, in the absence of added Ca2+ or in Ca2+-free solutions (0.5 mM EGTA), barium enters through the voltage-dependent channel and the contraction is reduced because the intracellular stores of calcium are limited.
Departamento de Farmacologia, Escola Paulista de Medicina, São Paulo, SP, Brasil.
E P
EP
Breuing
R
R
Kaminskas
Y L
YL
Kobashi
R P
RP
Markus
D000818:Animals; D001464:Barium / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D017609:Barium Compounds; D002118:Calcium / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002712:Chlorides; D006168:Guinea Pigs; D007537:Isometric Contraction; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects*; D009543:Nifedipine / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology*; D014649:Vas Deferens / Q000187:drug effects*
D017609:Barium Compounds; D002121:Calcium Channel Blockers; D002712:Chlorides; C024986:barium chloride; D001464:Barium; D012964:Sodium; D009543:Nifedipine; D002118:Calcium