Effects of Diltiazem on Guinea Pig Portal Vein in Hypertonic Solution

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Abstract
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. 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 Ca2+-free solution, the spontaneous contraction was completely abolished. In addition, the phasic contraction was suppressed by CoCl2. Apparenly, the spontaneous contractile activity of the guinea pig portal vein in hypertonic solution was myogenic in nature and Ca2+ played an essential role in the contraction. Diltiazem suppressed the spontaneous phasic contraction in isotonic and hypertonic solutions. The spontaneously generated and electrically evoked spikes in hypertonic solution were also inhibited by diltiazem. The effects of diltiazem on the mechanical activity and evoked spike were reversed by the addition of CaCl2. It is inferred that diltiazem suppresses the spontaneous phasic contraction by inhibiting the mobilization of Ca2+ which triggers the generation of the contraction.