Cyclic AMP and Cyclic GMP in Canine Peripheral Lung: RegulationIn Vivo1–4

Abstract

We examined regulation of lung function and basal concentrations of cyclic 3′,5′-adenosine monophosphate (AMP) and cyclic 3′,5′-guanosine monophosphate (GMP) in peripheral lung tissues in vivo in 64 anesthetized, paralyzed, and mechanically ventilated dogs. Lung samples were obtained through the open chest with clamps cooled in liquid nitrogen. We measured total lung resistance (Rl) and static lung compliance (Cl), which reflect central and peripheral airway function, respectively. Beta-adrenergic blockade (by intravenous administration of propranolol) after vagotomy, but not section of pulmonary sympathetic nerves, decreased cyclic AMP (−30 ± 5 per cent; mean ± SE), but did not affect Rl, Cl, or cyclic GMP. This decrease in cyclic AMP persisted despite added alpha-adrenergic blockade (by intravenous administration of phentolamine). Thus, under these experimental conditions, cyclic AMP in peripheral canine lung appears to be stimulated by circulating catecholamines acting on the beta-adrenergic receptor. However, the basal concentrations of cyclic AMP and cyclic GMP were unaffected by cholinergic blockade, blockade of H1- and H2-histamine receptors, or inhibition of prostaglandin synthetase. There were no direct correlations between changes in basal concentrations of cyclic nucleotides and peripheral lung function, which probably reflected the lack of basal airway smooth muscle tone during these experiments. In contrast, exogenous histamine induced changes in airway smooth muscle tone and cyclic nucleotides that correlated closely. Intravenous administration of 0.05 mg of histamine per kg of body weight to 6 vagotomized dogs during infusion of propranolol decreased Cl, decreased lung volume, increased Rl, and decreased blood pressure, followed by increases in cyclic AMP (104 ± 26 per cent) and cyclic GMP (157 ± 93 per cent). These changes were inhibited by chlorpheniramine, indicating that histamine acted on H1-receptors. The increase in cyclic AMP, but not the increase in cyclic GMP or the physiologic reaction, was inhibited by indomethacin, suggesting that the change in cyclic AMP was caused by prostaglandins or a precursor. These results were consistent with previous studies in vitro, which indicates that changes in concentrations of nucleotides probably do not initiate, but may modulate histamine-induced effects on peripheral lung. Because of the heterogeneity of peripheral lung, however, correlations between changes in cyclic nucleotides and lung function do not prove the existence of causal relationships. To relate changes in cyclic AMP and cyclic GMP to changes in function in vivo, other experimental approaches such as immunocytochemical localization of cyclic nucleotides will be required.