Desensitization and resensitization of β1- and putative β4-adrenoceptor mediated responses occur in parallel in a rat model of cardiac failure

1. Cardiostimulant effects of the non-conventional partial agonist, CGP 12177A, are mediated by a receptor distinct from the β ₃ -adrenoceptor and termed the putative β ₄ -adrenoceptor. Using a rat model of cardiac failure, induced by myocardial infarction (MI), we compared the desensitization and resensitization of responses to CGP 12177A with those to isoprenaline and RO 363 in left (LA) and right atria (RA). We also examined the ability of β-adrenoceptor antagonists to block responses to CGP 12177A. 2. MI reduced the maximum inotropic response to isoprenaline by 48% (sham 4.1 ± 0.6 mN, n = 10; MI 2.1 ± 0.4 mN, n = 8, P < 0.02), RO 363 by 61% (sham 4.2 ± 0.5 mN, n = 10 MI 1.8 ± 0.3 mN, n = 8, P < 0.005) and CGP 12177A by 49% (sham 1.4 ± 0.1 mN, n = 5; MI 0.7 ± 0.2 mN, n = 7 P < 0.O5 in electrically stimulated LA. MI also reduced the sensitivity to isoprenaline (pEC ₅₀ : sham 8.79 ± 0.08, n = 10, MI 8.30 ± 0.10, n = 8; P = 0.001) and RO 363 (pEC ₅₀ : sham 8.69 ± 0.07, n = 10; MI 8.33 ± 0.10, n = 8; P < 0.01). The maximum chronotropic responses to isoprenaline, RO 363 and CGP 12177A in RA were unaffected. 3. Pertussis toxin treatment (10 μg kg ^-1 , i.p.) restored the maximum inotropic response and sensitivity to isoprenaline (sham 3.5 ± 0.5 mN. n = 9; MI 3.2 ± 0.6 mN, n = 11, P = 0.702) and CGP 12177A (sham 1.6 ± 0.3 mN, n = 6; MI 1.9 ± 0.4 mN, n = 7, P = 0.537) in MI animals to levels similar to those in the sham group. 4. CGP 20712A (pK(B): LA 6.7 ± 0.2, n = 6; RA 7.1 ± 0.1, n = 4), ICI 118,551 (pK(B): LA 6.4 ± 0.1, n = 5; RA 6.3 ± 0.1, n = 6), propranolol (PK(B): LA 6.6 ± 0.1, n = 5; RA 6.8 ± 0.1, n = 6) and bupranolol (pK(B): LA 7.2 ± 0.1, n = 6; RA 7.7 ± 0.1, n = 8), showed moderate affinity for the putative β ₄ -adrenoceptor. 5. Desensitization after MI and resensitization (after pertussis toxin treatment) to isoprenaline and CGP 12177A therefore occur in parallel, suggesting that the β ₁ - and putative β ₄ -adrenoceptor use the same signalling pathway. Antagonist affinity studies confirmed that drugs acting at β ₁ -adrenoceptors also interact with putative β ₄ -adrenoceptors with approximately 100 times lower affinity. We suggest that CGP 12177A produces its cardiac effects by interacting with a low affinity state of the β ₁ -adrenoreceptor.