ANGIOTENSIN-INDUCED DESENSITIZATION OF THE PHOSPHOINOSITIDE PATHWAY IN CARDIAC-CELLS OCCURS AT THE LEVEL OF THE RECEPTOR

Previous studies show that angiotensin II (Ang II) increases phosphoinositide turnover in cultured neonatal heart cells. Ang II has also been shown to transiently increase spontaneous beating behavior in these cells. In this study we seek to identify the molecular mechanism underlying this rapid (3-5-minute) desensitization. Time-course studies on the accumulation of [H-3]inositol phosphates indicate that the loss in functional responsiveness correlates with reduced efficacy of Ang II to activate the phosphoinositide path. Binding studies with I-125-Ang II revealed that there was no change in surface receptor binding capacity during the time in which desensitization developed. Normal phosphoinositide and functional responses are observed when desensitized cells are treated with probes that activate the cardiac phosphoinositide pathway at discrete steps. These studies reveal that the functional status of the major components of the phosphoinositide signaling pathway, including G proteins, phospholipase C, and protein kinase C (PKC), are normal during maintained Ang II desensitization. To study the potential role of PKC in Ang II desensitization, the cells are treated with TPA for 24 hours, which downregulates PKC activity. PKC-depleted cells rapidly desensitize after Ang II application. We conclude that the selective Ang II-evoked biochemical/functional desensitization involves inhibition at the level of the receptor, rather than at a component downstream in the path, and that this process is independent of PKC and loss of surface binding capacity.