Measurement of cAMP in the cardiac conduction system of rats

To characterize differences in regional cAMP production in the cardiac conduction system, 18 rats were anesthetized with pentobarbital (65 mg/kg IP) and randomized into a control (n = 9) and a stimulated group (n = 9). The stimulated group received aminophylline (20 mg/kg SC) and isoproterenol (16 μg/kg SC). The concentration of cAMP in freeze-dried, micro dissected pieces (1-3 μg) of cardiac tissue was measured using a new microanalytical method. The cAMP contents in tight atrium, atrioventricular node, His bundle, and left ventricle (fmol/μg dry weight, mean ± SE) were 38.9 ± 2.5, 39.0 ± 4.3, 46.4 ± 6.1, and 41.4 ± 3.3 in controls and 72.9 ± 6.7, 86.1 ± 2.9, 115.0 ± 11.5, and 79.5 ± 7.3 in the stimulated group, respectively. Basal cAMP levels were similar throughout the heart, whereas isoproterenol increased cAMP levels in all regions (p<0.01). Furthermore, cAMP levels in His bundle, after isoproterenol, were higher than in any other region (p<0.05). These results demonstrate that: (a) cAMP can be measured in discrete portions of the cardiac conduction system; (b) there are significant regional differences of β-adrenergic control in the cardiac conduction system; and (c) cAMP production after β-adrenergic stimulation was lower than expected in the AV nodal region, based on previously described β- adrenoceptor density measurements.