Homologous desensitization of bombesin‐induced increases in intracellular Ca2+ in quiescent swiss 3T3 cells involves a protein kinase C‐independent mechanism

Abstract

Addition of bombesin to Swiss 3T3 cells causes a rapid and transient increase in the intracellular concentration of Ca²⁺ ([Ca²⁺]_i), which is followed by desensitization to a subsequent addition of the peptide. The concentrations of bombesin used to study this acute cellular desensitization (0.1–0.5 nM) did not deplete the intracellular pool of Ca²⁺ released by inositol(1,4,5)trisphosphate, as shown by addition of vasopressin after consecutive additions of bombesin. Two lines of evidence support the conclusion that activation of protein kinase C (PKC) does not mediate the acute homologous desensitization of Ca²⁺ responses induced by bombesin. First, long-term treatment (48 h) of Swiss 3T3 cells with phorbol 12, 13-dibutyrate (PDB) to deplete PKC did not prevent homologous desensitization. The responses to second additions of bombesin at 0.1, 0.25, and 0.5 nM were 42%, 26% and 11% of the initial responses, respectively. Second, the PKC inhibitor GF 109203X did not alter homologous desensitization at concentrations that completely prevented the inhibition of Ca²⁺ mobilization induced by PDB and blocked PDB-mediated phosphorylation of the prominent PKC substrate 80K/MARCKS. We conclude that acute homologous desensitization of Ca²⁺ responses induced by bombesin occurs through a PKC-independent mechanism.