GTP-Binding protein regulates the contractile response elicited by the phorbol ester (PMA)-induced activation of protein kinase C in the isolated rat aorta

1. The aim of the present study was to investigate the involvement of GTP-binding protein in the contractile response induced by activation of protein kinase C (PKC) in isolated rat aorta. The rats were treated with islet activating protein (IAP) for 4 days prior to the experiments. 2. In the aorta from control rats, phorbol 12-myristate 13-acetate (PMA) produced biphasic contractions. The twitch contraction superimposed on the slowly developing contraction. The twitch contraction was abolished by the removal of external Ca2+ or by treatment with nicardipine. In the aorta pretreated with IAP, PMA produced only a slowly developing contraction, and no twitch contraction was induced. 3. The application of Ca2+ to aortic strips in a Ca2+-free solution, that had been treated with 10(-6) M PMA caused concentration dependent contraction, and the contraction was completely inhibited by IAP. 4. Pretreatment with IAP inhibited Ca2+-induced contraction of the aorta in Ca2+-free medium in the presence of 10(-6) M clonidine, but did not affect the Ca2+-induced contraction in the medium treated with 10(-6) M phenylephrine and 10(-7) M nicardipine. 5. These results suggest that the activation of PKC by PMA produces biphasic contractions in the rat aorta. The twitch contraction may be induced by the activation of voltage-dependent Ca2+ channels. and the activation may be regulated by IAP-sensitive GTP-binding protein.