EFFECTS OF DILTIAZEM ON GUINEA PIG PORTAL VEIN IN HYPERTONIC SOLUTION

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Effects of diltiazem were investigated on the isolated portal vein of the guinea pig in hypertonic solution. Hyperosmolarity was produced by sucrose. In contrast to a small and somewhat irregular spontaneous contraction in isotonic solution, the portal vein in hypertonic solution elicited a spontaneous contraction at regular intervals and with a large amplitude and low frequency. The spontaneous phasic contraction in hypertonic solution was not affected by tetrodotoxin, atropine and propranolol. Phentolamine also had no significant influence in some preparations. On the contrary, remarkable changes in the contractile activity were found when the extracellular concentration of CaCl2 was reduced from 2.5 to 1.3 or 0.6 mM. In calcium-free solution, the spontaneous contraction was completely abolished. In addition, the phasic contraction was suppressed by CoCl2. Thus, it is assumed that the spontaneous contractile activity of the guinea pig portal vein in hypertonic solution is myogenic in nature and Ca++ plays an essential role in the contraction. On the other hand, diltiazem suppressed the spontaneous phasic contraction in both isotonic and hypertonic solutions. The spontaneously generated and electrically evoked spikes in hypertonic solution were also inhibited by diltiazem. Furthermore, the effects of diltiazem on the mechanical activity and evoked spike were reversed by the addition of CaCl2. Therefore, it is inferred that diltiazem suppresses the spontaneous phasic contraction by inhibiting the mobilization of Ca++ which triggers the generation of the contraction.
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EFFECTS OF DILTIAZEM ON GUINEA PIG PORTAL VEIN IN HYPERTONIC SOLUTION

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