Injections of cyclic AMP (cAMP) or cyclic GMP (cGMP) into identifiable gastropod neurones under voltage clamp induced reversible cytoplasmic pH changes which were measured using the indicator dye Arsenazo III and pH micro‐electrodes. Similar injections of 5' AMP or 5' GMP did not induce such effects. In about one‐half of the population of neurones examined, cAMP injections (0.1‐10 mM) induced pH decreases with latencies of 1‐5 min and maximum response times 12‐25 min post‐injection. Comparable injections of cGMP into these cells resulted in decreases with latencies less than 1 min and maximum response times 5‐10 min. The amplitude and duration of these pHi decreases were dose dependent. In the other half of the population of neurones tested, cAMP injection induced an immediate alkalinization lasting 5‐10 min followed by an acidification which displayed a maximum response time within the same range as those in the first group. cGMP injection into these cells induced only acidifications with faster peak responses than with cAMP. Since cGMP did not elicit alkalinizing responses, the slower response time of the first group of cells with cAMP may reflect an always present underlying alkalinization. The nucleotide‐induced acidification was potentiated in neurones bathed in the phosphodiesterase inhibitor IBMX. Injections of non‐hydrolysable cAMP analogues also induced acidifications which were longer lasting than comparable doses of cyclic AMP. These results indicate that the elevated [H+]i was not simply due to hydrolysis of the injected cyclic nucleotide. The pHi changes were invariably of much longer duration than a nucleotide‐induced inward current (Connor & Hockberger, 1984) and persisted in Na‐free salines which eliminated the current response. These results support the notion that cyclic nucleotide elevation can affect cellular metabolic processes distinct from effects on ion transport mechanisms.
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