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)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 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., 423:140-149). 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 InsP₃. 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.