Contribution of response kinetics to the response pattern: studies of responses to thyrotropin-releasing hormone in Xenopus oocytes

Inverse agonist abolishes desensitization of a constitutively active mutant of thyrotropin-releasing hormone receptor: role of cellular calcium and protein kinase C

1. C335Stop is a constitutively active mutant of the TRH receptor (TRH-R). To investigate the mechanism of the decreased responsiveness of C335Stop TRH-R, we studied cellular Ca2+ concentrations ([Ca2+]i) in AtT20 cells stably transfected with C335Stop TRH-R cDNA, or Ca2+-activated chloride currents in Xenopus laevis oocytes expressing this mutant receptor after injection of cRNA. The competitive TRH-R binding antagonist, chlorodiazepoxide (CDE), was used as an inverse agonist to study the contribution of constitutive activity to desensitization. 2. Acute treatment with CDE resulted in a rapid (within minutes) decrease in [Ca2+]i and an increase in the response amplitude to TRH with no measurable change in receptor density. Conversely, removal of chronically administered CDE caused a rapid increase in [Ca2+]i and a decrease in TRH response amplitude. 3. CDE abolished heterologous desensitization induced by C335Stop TRH-R on muscarinic m1-receptor (ml-R) co-expressed in Xenopus oocytes. 4. Chelation of extracellular calcium with EGTA caused a rapid decrease in [Ca2+]i and a concomitant increase in the response to TRH in AtT20 cells expressing C335Stop TRH-Rs. 5. Chelerythrine, a specific inhibitor of protein kinase C (PKC), reversed the heterologous desensitization of the response to acetylcholine (ACh). The phosphoserine/phosphothreonine phosphatase inhibitor, okadaic acid, abolished the effect of chelerythrine. 6. Down-regulation of PKC by chronic exposure to phorbol 12-myristate 13-acetate (PMA) or acute inhibition with chelerythrine caused a partial resensitization of the response to TRH. 7. Western analysis indicated that the alpha subtype of protein kinase C was down-regulated in cells expressing C335Stop TRH-Rs. Following a 5 min exposure to PMA, the residual alphaPKC translocated to the particular fraction. 8. We propose that cells expressing the constitutively active mutant TRH-R rapidly desensitize their response, utilizing a mechanism mediated by an increase in [Ca2+]i and PKC.

Alkaline pH facilitates the exchange of guanine nucleotides: a possible mechanism for modulation of the kinetics of responses mediated by guanine nucleotide-binding proteins

GTP gamma S1 binding experiments in a particulate preparation from Xenopus oocytes revealed two binding sites at pH = 6.9: A high affinity site (Kd = 77 +/- 4 nM) and a low affinity site (Kd = 8.74 +/- 0.05 microM). Alkaline pH (8.5) caused a significant increase in the dissociation constants of both sites (160 +/- 46 nM and 30.7 +/- 1.6 microM, respectively). In purified plasma membrane preparation, alkaline pH increased the rate of dissociation of GTP gamma S. We have previously proposed that the activation of a G-protein by the agonist-occupied receptor is rate-limiting in the kinetics of hormone-induced responses (Lipinsky et al., 1993; Pflugers Arch., 425:140-149). We have, therefore, assayed the latencies of responses evoked by TRH at different pH, in oocytes expressing the TRH receptor. A change in the medium pH was reflected by an approximately tenfold smaller change in cellular pH (pHi). Alkalinization of the medium (from pH 7.4 to 8.5) caused a shortening of latency (by 45%), whereas acidification to pH = 6.0 prolonged it (by 87%). Moreover, alkalinization decreased the latency and increased the rate of responses to microinjected GTP gamma S, but did not change the latency of responses to microinjected InsP3. These results show that activation of plasma membrane receptors coupled to G-proteins, concurrent with a change in pHi, can alter the kinetic pattern of physiological responses, thus affecting the ultimate physiological output of the cell. This finding suggests that a change of pH, is a novel potential mechanism for modulation of responses mediated by G-proteins.