Adenosine induces prolonged anti‐β‐adrenergic effects in guinea‐pig papillary muscle

A sustained anti‐β‐adrenergic effect of adenosine has been reported. This study was initiated to investigate this topic and especially elucidate the role of protein kinase C (PKC). Contractile force amplitude and action potential duration at 90% repolarization (APD₉₀) were measured in guinea‐pig papillary muscles before and after 5 min challenge with 5 nm isoproterenol. Protocols contained 30 min exposure to the test agents adenosine 33 μm (ado), adenosine + PKC‐inhibitor bisindolylmaleimide 20 nM (ado + BIM), PKC‐activator 1,2‐dioctanoyl‐sn‐glycerol 10 μm (DOG) and α‐agonist phenylephrine 5 μm (phe). Isoproterenol was given at the end of test exposure and after 15 min washout. Results are mean ± SEM of percentage‐change, P ≤ 0.05 considered significant and labelled *. The first isoproterenol challenge significantly increased contractile force (27 ± 7%*) in the control group. Responses in the test groups were 2 ± 4 (ado), 1 ± 5 (ado + BIM), 14 ± 4* (DOG), 0 ± 2% (phe). After washout of adenosine, DOG and phenylephrine, isoproterenol induced 3 ± 8 (ado), 23 ± 5* (ado + BIM), 13 ± 5* (DOG), 15 ± 7% (phe) increase in test groups compared with 22 ± 5%* increase in contractile force in the control group. After 45 min washout of adenosine the inotropic response was still significantly reduced compared with control (29 ± 4 vs. 79 ± 8%*). Isoproterenol stimulation shortened APD₉₀ in controls at both time points (5 ± 1%* and 4 ± 1%*), with no significant shortening in test groups. Adenosine induces sustained anti‐β‐adrenergic effects on contractile force as well as APD₉₀. A role for PKC in signal transduction is supported with respect to contractile force.