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 γ S 1 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 μM). Alkaline pH (8.5) caused a significant increase in the dissociation constants of both sites (160 ± 46 nM and 30.7 ± 1.6 μM, respectively). In purified plasma membrane preparation, alkaline pH increased the rate of dissociation of GTP γ 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 (pH i ). 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 γ S, but did not change the latency of responses to microinjected InsP 3 . These results show that activation of plasma membrane receptors coupled to G‐proteins, concurrent with a change in pH i , 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 i is a novel potential mechanism for modulation of responses mediated by G‐proteins. © 1996 Wiley‐Liss, Inc.