Measurement of adenylylcyclase activity in the AV nodal region of the canine heart: Evidence for inhibition by adenosine and acetylcholine

Although it is essential to cardiac conduction, little is known about the biochemistry underlying postreceptor adrenergic, cholinergic and purinergic processes in the AV node. To study these mechanisms, we adapted a new and highly sensitive fluorometric assay for cyclic adenosine monophosphate (AMP) to characterize regional adenylylcyclase activity (cyclic AMP production in pmol/min/mg of protein) in membrane preparations made from 20-50 pieces of freeze-dried, 20-μn thick, microdissected samples of tissue from canine tight atrium, the AV nodal region, and left ventricle. Basal and NaF-stimulated adenylylcyclase activity (mean ± SEM, n = 6) were 7.2 ± 0.4 and 72.4 ± 7.5 in atrial, 15.6 ± 1.3 and 58.8 ± 4.7 in AV nodal, and 6,4 ± 0.9 and 66.7 ± 5.0 in ventricular tissues, respectively. Isoproterenol (10^-7-10^-4 M) increased adenylylcyclase activity in a dose-dependent fashion in three different regions. The isoproterenol (10^-6 M)-stimulated adenylylcy]clase activity (n=6) was 14.4 ± 1.3 in atrial, 21.9 ± 1.6 in AV nodal and 13.4 ± 1.4 in ventricular tissues. Adenosine (10^-3 M) and carbachol (10^-5 M) inhibited isoproterenol (10^-6 M)-stimulated adenylylcyclase activity to 10.1 ± 1.1, 12.9 ± 1.3 in atrial, 15.1 ± 1.6, 15.5 ± 1.2 in AV nodal, and 7.5 ± 0.7, 11.9 ± 1.2 in ventricular tissues, respectively. The results demonstrate that there are regional differences in adenylylcyclase activity under basal conditions and after adrenergic, purinergic, and cholinergic stimulation in the heart. Unlike adenosine, the inhibitory effects of cholinergic stimulation appear to be more specific for the AV node.