The objective this investigation was to determine the relative importance of type I, III, and IV phosphodiesterases in the regulation of cyclic adenosine monophosphate (cAMP) in the renal circulation. In the first experimental series, four groups of isolated rat kidneys perfused with Tyrode's solution were stimulated with isoproterenol (3 microM) and then treated with increasing concentrations (from the approximately IC50 to 30 times the approximately IC50 in threefold increments) of one of four phosphodiesterase inhibitors: group 1, 3-isobutyl-1-methylxanthine, a "broad-spectrum" phosphodiesterase inhibitor (10-300 microM); group 2, Ro 20-1724, a selective type IV phosphodiesterase inhibitor (3-100 microM); group 3, 8-methoxymethyl-3-isobutyl-1-methylxanthine, a selective type I phosphodiesterase inhibitor (3-100 microM); and group 4, milrinone, a selective type III phosphodiesterase inhibitor (0.3-10 microM). In the second experimental series, five groups of cultured preglomerular (interlobular and afferent arteriolar) vascular smooth-muscle cells were stimulated with isoproterenol (1 microM) and treated with vehicle or supramaximal concentrations (30 times IC50) of either 3-isobutyl-1-methylxanthine (300 microM), Ro 20-1724 (100 microM), 8-methoxymethyl-3-isobutyl-1-methylxanthine (100 microM), or milrinone (10 microM). In perfused kidneys and cultured pre-glomerular vascular smooth-muscle cells, 3-isobutyl-1-methylxanthine and Ro 20-1724 similarly increased renal cAMP release and total cellular (extracellular + intracellular) cAMP levels, respectively. In contrast, neither 8-methoxymethyl-3-isobutyl-1-methylxanthine nor milrinone affected renal cAMP release or total cellular cAMP levels. These data indicate that in the renal circulation, type IV phosphodiesterase is the predominant phosphodiesterase isozyme.