The effect of parathyroid hormone-related protein (PTHrP) on cytosolic free Ca2+ level ([Ca2+]i) and tension in rat aortic smooth muscle was investigated with special reference to the role of production and action of cyclic AMP. Rat aortic spiral strip preparations without endothelium were treated with the acetoxymethyl ester of fura 2, and the ratio of fluorescences (R340/380), an index of [Ca2+]i, emitted from smooth muscle, was measured. The tension of the preparations was simultaneously measured. PTHrP-(1-34) produced concentration-dependent decreases both in the tension and in R340/380 increased by phenylephrine (10(-7) M). These effects were significantly inhibited by pretreatment with either PTHrP-(7-34) (10(-6) M), a PTHrP receptor antagonist, or with Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (RpcAMPS; 10(-4) M), a cyclic AMP-dependent protein kinase inhibitor. Dibutyryl cyclic AMP (10(-5)-10(-3.5 M) elicited effects similar to those of PTHrP-(1-34). PTHrP-(1-34) was found to significantly elevate aortic cAMP level, measured by specific radioimmunoassay, after 5 min incubation with PTHrP-(1-34). These results suggest that the decrease in [Ca2+]i is involved in the vasodilator action of PTHrP, and that the decreases both in tension and in [Ca2+]i might be attributed to cyclic AMP production stimulated by PTHrP.