Differences between the third cardiac β‐adrenoceptor and the colonic β3‐adrenoceptor in the rat

1 The heart of several species including man contains atypical β‐adrenoceptors, in addition to coexisting β1‐ and β2‐adrenoceptors. We now asked the question whether or not the third cardiac β‐adrenoceptor is identical to the putative β3‐adrenoceptor. We compared the properties of the third cardiac β‐adrenoceptor with those of β3‐adrenoceptors in isolated tissues of the rat. To study the third cardiac β‐adrenoceptor we used spontaneously beating right atria, paced left atria and paced left ventricular papillary muscles. As a likely model for putative β3‐adrenoceptors we studied atypical β‐adrenoceptors of the colonic longitudinal muscle precontracted with 30 mM KCl. We used β3‐adrenoceptor‐selective agonists, antagonists and non‐conventional partial agonists (ie high‐affinity blockers of both β1‐ and β2‐adrenoceptors known to exert also stimulant effects through β3‐adrenoceptors). 2 The non‐conventional partial agonist (−)−CGP 12177 caused positive chronotropic effects in right atria (pD2 = 7.3) and positive inotropic effects in left atria (pD2 = 7.5). The stimulant effects of (−)−CGP 12177 were resistant to blockade by 200 nM‐2μm (−)−propranolol and 3 μm ICI 118551 (a β2‐selective antagonist) but antagonized by 1 μm (−)−bupranolol (pKB = 6.4–6.8), 3 μm CGP 20712A (a β1‐ selective antagonist) (pKB = 6.3–6.4) and 6.6 μm SR 59230A (a β3‐selective antagonist, pKB = 5.1–5.4). 3 The non‐conventional partial agonist cyanopindolol caused positive chronotropic effects in right atria (pD2 = 7.7) and positive inotropic effects in left atria (pD2 = 7.1). The stimulant effects of cyanopindolol were resistant to blockade by 200 nM (−)−propranolol but antagonized by 1 μm (−)−bupranolol (pKB = 6.8‐7.1). 4 Neither (−)−CGP 12177 nor cyanopindolol caused stimulant effects in papillary muscles at concentrations between 0.2 nM and 20 μm. 5 In the presence of 200 nM (−)−propranolol the β3‐adrenoceptor‐selective agonists BRL 37344 (6 μm), SR 58611A (6 μm), ZD 2079 (60 μm) and CL 316243 (60 μm) did not cause stimulant effects or modify the potency and efficacy of the effects of (−)−CGP 12177 in right and left atria. The combination of 2 μm (−)−propranolol and 2 μm (−)−noradrenaline did not modify the chronotropic potency and efficacy of (−)−CGP 12177 compared to the potency and efficacy in the presence of 2 μm (−)−propranolol alone. 6 (−)−CGP 12177 relaxed the colon with a pD2 of 6.9 and a maximum effect of 55% compared to (−)−isoprenaline. The relaxant effects of (−)−CGP 12177 were resistant to blockade by 200 nM (−)−propranolol, 3 μm CGP 20712A, 3 μm ICI 118551 but blocked by 2 μm (−)−propranolol (pKB = 6.0), 1 μm (−)−bupranolol (pKB = 6.4) and 3 μm SR 59230A (pKB = 6.3). In the presence of 200 nM (−)−propranolol, (−)−CGP 12177 (20 μm) antagonized surmountably the relaxant effects of BRL 37344 (pKP = 7.3), (−)−noradrenaline (pKP = 7.0); and CL 316243 (pKP = 7.0). 7 Cyanopindolol in the presence of 200 nM (−)−propranolol relaxed the colon with a pD2 of 7.0 and a maximum effect of 40% compared to (−)−isoprenaline. As expected from a partial agonist, cyanopindolol antagonized the relaxant effects of both BRL 37344 and CL 316243 with a pKP = 7.6 and (−)−noradrenaline with a pKP = 7.4. 8 The following β3‐adrenoceptor‐selective agonists were potent colonic relaxants (pD2 values between parentheses): BRL 37344 (9.1), ZD 2079 (7.0), CL 316243 (9.0) and SR 58611A (8.2). The relaxant effects of these agonists were only marginally affected by 200 nM (−)−propranolol, not blocked by 3 μm CGP 20712A or 3 μm ICI 118551, and blocked by SR 59230A 3 μm (pKB = 6.9‐7.5), 1 μm (−)−bupranolol (pKB = 6.2–6.4) and 2 μm (−)−propranolol (pKB = 6.3–6.5). 9 The colonic relaxation caused by the nanomolar concentrations of the β3‐adrenoceptor‐selective agonists and the non‐conventional partial agonists (−)−CGP 12177 and cyanopindolol and their relative resistance to blockade by antagonists with high affinity for β1‐ and β2‐adrenoceptors but blockade by the β3‐adrenoceptor selective SR 59230A agree with the hypothesis that the receptors involved are β3‐adrenoceptors. On the other hand, the failure of micromolar concentrations of β3‐adrenoceptor‐selective agonists to produce cardiac stimulation or affect the cardiostimulant effects of (−)−CGP 12177 is inconsistent with the hypothesis that the third cardiac β‐adrenoceptor is β3. Additionally, the selective blockade of the colonic putative β3‐adrenoceptor compared to the third cardiac β‐adrenoceptor by SR 59230A, as well as the blockade of cardiac but not colonic receptors by CGP 20712A is also inconsistent with an identical putative β3‐adrenoceptor in colon and heart. We conclude that in the rat the third cardiac β‐adrenoceptor is different from the colonic β3‐adrenoceptor.