The normal function of retinal capillaries to distribute blood within the retina depends on appropriate contractility of retinal pericytes, which is thought to be modulated by agents that alter intracellular cyclic adenosine-3'-monophosphate (cAMP) levels. We examined the hypothesis that the vasoactive peptides Vasoactive Intestinal Peptide (VIP) and Pituitary Adenylate Cyclase Activating Peptide (PACAP) reduce pericyte contractility via a protein kinase A (PKA)-mediated intracellular pathway that utilises cAMP. We utilised a single-call assay of contractility that is based on visualising the contractile force exerted by the pericytes on a silicone elastomer substrate and quantified, as a contractility index, from the number and length of wrinkles induced in the silicone elastomer by the pericytes. Pericytes were cultured from the retinas of freshly killed abattoir cattle, and identified in culture using immunohistochemical techniques. The pericytes contracted in response to norepinephrine (EC(50)=8 microM) and relaxed in response to both VIP (EC(50)=48 nM) and PACAP (EC(50)=3 nM). The relaxation induced by PACAP was inhibited by Rp-cAMPS (EC(50)=26 microM), which is an agent that inhibits cAMP binding at PKA. We confirmed the activation of PKA by PACAP in experiments where H89 also inhibited the PACAP-induced relaxation. U71322, which inhibits phospholipase C-linked events, was also able to inhibit the PACAP-induced pericyte relaxation. Our results support the hypothesis that PACAP leads to the relaxation of pericytes via a PKA-mediated intracellular pathway and a phospholipase C-mediated pathway, which probably relies on hyperpolarisation because of activation of Ca(2+)-dependent potassium channels. This single-cell assay has proved useful as the basis for the development of a diagnostic procedure for diabetic retinopathy, which is an eye disease caused by abnormal regulation of blood flow in the retinal capillaries.