Protein kinase C contributes to desensitization of ANG II signaling in adult rat cardiac fibroblasts.

We have studied G(q)-linked ANG II signaling [inositol phosphate (IP) accumulation, Ca(2+) mobilization] in primary cultures of rat cardiac fibroblasts (CFs) and have found that ANG II initiates a protein kinase C (PKC)-mediated negative feedback loop that rapidly terminates the ANG II response. Pharmacological inhibition of PKC by staurosporine and GF-109203X doubled IP production over that achieved in response to ANG II alone. Inhibition of PKC also led to larger Ca(2+) transients in response to ANG II, suggesting that Ca(2+) mobilization was proportional to G(q)-phospholipase C-IP(3) activity under the conditions studied. Depletion of cellular PKC by overnight treatment with phorbol 12-myristate 13-acetate (PMA) similarly augmented ANG II-induced IP production. Acute activation of PKC by PMA halved IP formation, with an EC(50) approximately 1 nM; 4alpha-PMA was inactive. Time course data demonstrated that ANG II-mediated IP production fully desensitized within 30 s; PKC inhibition reduced the rate and extent of this desensitization. In cells desensitized to ANG II, a purinergic agonist still mobilized intracellular Ca(2+), indicating that desensitization was homologous. The ANG II-induced Ca(2+) signal was fully resensitized within 30 min. The data demonstrate that a large portion of the IP-Ca(2+) responses of rat CFs to ANG II are short-lived because of rapid, PKC-mediated desensitization.