Supersensitivity to β‐adrenoceptor stimulation evoked in cultured neonatal rat heart myocytes by L(+)‐lactate and pyruvate

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Abstract
1. Cells from the ventricles of newborn rats were cultured for 8 days in flasks attached to a rocker apparatus to ensure an adequate oxygen supply. 2. The rocked cultures, which had previously been found to be low in lactate and subsensitive to the positive chronotropic action of the beta-adrenergic agonist isoprenaline (ISO; EC50 of (+/-)-ISO around 7 x 10(-7) M), became resensitized and even highly supersensitive to the catecholamine upon treatment with 3 mM L(+)-lactate or 1 mM pyruvate. 3. The resulting concentration-response curves were anomalous in that they extended over 8 log units, with a threshold at about 10(-13) M, but with little or no change in the height and the position of the maximum (at 10(-5) M). The EC50 values and 95% confidence intervals were 2.4 (1.9-3.0) and 5.4 (4.9-6.0) x 10(-11) M, respectively, for the lactate- and pyruvate-induced components of the chronotropic response to (+/-)-ISO. 4. The supersensitive portion of the ISO concentration-response curve was abolished by (-)-propranolol (10(-6) M), indicating that it was due to beta-adrenoceptor stimulation. 5. The cultured heart cells had to be incubated with L(+)-lactate or pyruvate for a minimum of 45 min before an increase in sensitivity to ISO became apparent. This latency was not due to a requirement for protein synthesis. 6. The adenosine-3',5'-monophosphate (cAMP) response to ISO was not noticeably altered by lactate, but (+/-)-ISO, which at 10(-8) M had no effect on the activation state of cAMP-dependent protein kinase (PKA), caused a significant increase in the activity of the enzyme following a 2-h exposure of the cells to 3 mM L(+)-lactate. 7. alpha-cyanocinnamate, an inhibitor of transmembrane transport of lactate and pyruvate, severely inhibited the utilization of L-[U-14C] lactate by the cultured cells at a concentration (5 microM) that eliminated the lactate-evoked potentiation of the chronotropic action of ISO without significantly affecting its unpotentiated action. 8. The beat-accelerating action of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMX) and the lipophilic N6,2'-O-dibutyryl derivative of cAMP (dbcAMP), both of which are capable of elevating myocardial cAMP levels, was not potentiated by 1 mM pyruvate. 9. The question is raised, whether accumulation of lactate, a biochemical hallmark of anaerobiosis, might be a factor in some of the catecholamine-triggered events occurring in acute myocardial ischaemia and infarction.