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ACTIONS OF PROSTAGLANDIN F2α AND NORADRENALINE ON CALCIUM EXCHANGE AND CONTRACTION IN RAT MESENTERIC ARTERIES AND THEIR SENSITIVITY TO CALCIUM ENTRY BLOCKERS

1 The actions of prostaglandin F2α (PGF2α) and noradrenaline on contraction and 45Ca exchange have been studied in rat mesenteric arteries. 2 PGF2α and noradrenaline contracted rat isolated mesenteric artery preparations to about the same extent. The PGF2α‐stimulated contractions, unlike those produced by noradrenaline, were completely inhibited in calcium‐free physiological solution. 3 The calcium entry blocking drugs, cinnarizine and flunarizine, had little effect on the resting exchange of calcium in the arterial smooth muscle, but inhibited PGF2α‐stimulated contractions and 45Ca uptake to a similar extent. 4 Flunarizine was about 7 fold more potent as an inhibitor of noradrenaline‐ than of PGF2α‐mediated contraction and 45Ca uptake and this ratio was about 50 for cinnarizine. 5 EGTA (1.25 mm) produced a relaxation of noradrenaline and PGF2α‐induced maximal contractions. Measured over the first 2 min of EGTA contact, the rate of relaxation was much faster in noradrenaline than in PGF2α‐stimulated preparations. 6 Turnover of cellular calcium (influx plus efflux) during the first 2 min of noradrenaline contact was much greater than that produced by PGF2α, largely due to a greater effect of noradrenaline on calcium efflux. 7 The results suggest that PGF2α‐ but not noradrenaline‐induced contractions are entirely dependent on the influx of extracellular calcium and that the agonists may stimulate calcium gating mechanisms differently.