Prostaglandin E1 potentiation of the spontaneous phasic contraction of rat isolated portal vein by a cyclopiazonic acid‐sensitive mechanism

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Abstract
The effect of prostaglandin E1 (PGE1) on the spontaneous phasic contraction of the rat isolated portal vein was studied. The isolated portal vein exhibited spontaneous phasic contractions. Removal of Ca2+ from Krebs‐Ringer solution or application of nifedipine abolished the spontaneous contraction, indicating that the contraction depends exclusively on Ca2+ influx through L‐type Ca2+ channels. On the other hand, cyclopiazonic acid (CPA), a specific inhibitor of Ca2+‐ATPase of sarcoplasmic reticulum (SR) increased the amplitude of the contractions, suggesting that the SR regulates the spontaneous contractions negatively by sequestration of Ca2+ entering through L‐type Ca2+ channels and buffering the rise in cytosolic Ca2+. PGE1 increased the amplitude of the spontaneous contraction in a concentration‐dependent manner without affecting the resting tension. The effect was completely abolished by nifedipine. Bay K 8644 and phenylephrine (PE) also increased the amplitude of the contraction in a concentration‐dependent manner. PGE1 at a concentration of 1 μm, Bay K 8644 at 100 nm and PE at 30 nm doubled the amplitude, respectively. Pretreatment with 1 μm CPA abolished the effect of PGE1, but the effects of Bay K 8644 and PE were not inhibited by pretreatment with CPA. In contrast, 10 μm ryanodine attenuated the effect of PE without affecting the contractile effect of PGE1. When the SR was depleted of Ca2+ by repeated applications of caffeine in a nominally Ca2+‐free Krebs‐Ringer solution, it took about 120 s to restore the spontaneous contraction after addition of Ca2+ into the solution. In CPA‐treated veins, the time taken to restore the contraction was shortened significantly. Pretreatment with 1 μm PGE1 shortened the time to the same extent as pretreatment with CPA did. These results suggest that PGE1 increases the amplitude of the spontaneous phasic contraction by a different mechanism from those by which PE and Bay K 8644 increase it. Inhibition of Ca2+‐ATPase of the SR might be involved in the vasoactive effect of PGE1. British Journal of Pharmacology (1997) 120, 1419–1426; doi:10.1038/sj.bjp.0701051