Ca2+‐DEPENDENT DIFFERENTIAL DEVELOPMENT OF CARBACHOL‐INDUCED DESENSITIZATION TO RECEPTOR AGONISTS AND HIGH K+ IN GUINEA‐PIG TAENIA CAECI

SUMMARY 1 Carbachol (CCh)-induced desensitization to CCh was interrupted by a transient resensitization during its early stage, with concomitant changes at the muscarinic receptor/G-protein level in smooth muscle of guinea-pig taenia caeci. To assess whether such a peculiar desensitizing process may heterologously regulate smooth muscle contraction, we examined the developmental processes of CCh-induced desensitization to histamine and high K+ and compared it with that to CCh. 2 Under Ca2+-containing physiological conditions, treatment with 10−4 mol/L CCh for 30 min induced heterologous desensitization to histamine and high K+. The development of desensitization to histamine was interrupted by a transient resensitization at 1 min in a manner similar to that to CCh. In contrast, CCh-induced desensitization to high K+ reached a peak at 1 min and was followed by a gradual resensitization up to a partial restoration at 30 min. 3 Under Ca2+-free conditions containing 0.2 mmol/L EGTA, treatment with 10−4 mol/L CCh for 30 min failed to induce heterologous desensitization to either histamine or high K+, whereas the CCh treatment developed homologous desensitization to CCh in a simple time-dependent manner without a resensitization phase. 4 These results suggest that cellular responsiveness to receptor agonists and non-receptor-mediated depolarizing stimulation is differentially regulated by Ca2+-dependent heterologous desensitization in smooth muscle.