The relationship of KCl‐ and prostaglandin F2α‐mediated increases in tension of the porcine coronary artery with changes in intracellular Ca2+measured with fura‐2

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Abstract
Changes in intracellular free Ca2+concentration ([Ca2+]i) were measured simultaneously with changes in muscle tension by use of the fluorescent Ca2+indicator, fura‐2, in coronary arterial rings of the Pig. Changes in [Ca2+]i were measured by monitoring the ratio of fluorescence due to excitation at 340 nm (F340) to that at 380 nm (F380). Increases in tension of the porcine coronary artery induced by prostaglandin F2α (PGF2α) (2–30 μm) and KCl (25–70 mm) were accompanied by increases in the F340/F380 fluorescence ratio of fura‐2. KCl‐induced increases in muscle tension, equivalent to those produced by PGF2α, were observed to occur in the presence of a higher [Ca2+]i. Nearly complete relaxation of KCl‐induced contractions by sodium nitroprusside (SNP) was accompanied by only a partial reversal of the increase in [Ca2+]i that occurred during contraction. Complete relaxation of the PGF2α‐contracted coronary artery by cromakalim (BRL 34915) was accompanied by a nearly complete reversal of the increase in [Ca2+]i caused by the contractile agent. The contractile state of smooth muscle is not an indicator of [Ca2+]i. The [Ca2+]itension relationship is dependent upon the type of pharmacological agent that is used to change muscle tension.