E ffects of peripheral benzodiazepine receptor modulating drugs, Ro 5-4864 and PK 11195, on tension induced by K+ and the calcium agonist SDZ 202 791 (S isomer), were studied in rat caudal arteries. A significant reduction of tonic phase tension occurred with 30 nM PK 11195 or 3 microM Ro 5-4864, but decreases of the initial (first 3 min), phasic contraction were detected only at the highest concentrations of Ro 5-4864 and PK 11195. Protoporphyrin IX, the putative endogenous ligand of the peripheral benzodiazepine receptor, (at 10-100 nM) markedly increased the effectiveness of Ro 5-4864 and PK 11195 in reducing phasic contraction. Intracellular calcium localization and distribution in fura-2 loaded single vascular cells were quantitated using a high sensitivity, two-stage microchannel plate, photon-counting (PMI-VIM) camera. Peripheral benzodiazepines reduced intracellular calcium release from centrally located calcium pools, and this decrease of calcium release was potentiated by protoporphyrin IX. The decrease in intracellular calcium activity, which was more pronounced in the central regions where sarcoplasmic reticular elements are numerous, was probably the major mechanism of these vasodilator properties. Measurements of soluble guanylate cyclase activity also supported the intracellular Ca2+ release mechanism. Under conditions where protoporphyrin IX did not significantly stimulate guanylate cyclase, Ro 5-4864 alone or more effectively in combination with protoporphyrin IX stimulated cGMP production and caused relaxation. Guanylate cyclase forms a possible target for these benzodiazepine modulators, a hypothesis that merits further investigation.