Milrinone Inhibits Sympathetic‐Mediated Tachycardia by a Postjunctional Action Independent of Cyclic AMP

In pithed rats with stimulated sympathetic outflow, the phosphodiesterase inhibitor milrinone (0.3 mg/kg, i.v.) decreased the peak tachycardic response produced by both sympathetic nerve stimulation (15 s at 0.5–3 Hz) and norepinephrine administration (0.3–5 μg/kg, i.v.). However, another phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 0.5 mg/kg, i.v.) had no effect on the peak tachycardic response to sympathetic stimulation. Similarly, in isolated rat atria, milrinone (9 μmol/L) inhibited the tachycardia produced by norepinephrine, whereas IBMX (1 μmol/L) had no effect. The inhibitory effect of milrinone on sympathetic responses was not due to changes in norepinephrine release since milrinone (9 μmol/L) increased norepinephrine release in isolated rat atria incubated with [3H]norepinephrine. When the duration of the tachycardia (rather than the peak tachycardic response) produced by sympathetic nerve stimulation was measured, it was found to be prolonged by both milrinone and IBMX, suggesting that in this case cyclic AMP was involved. Furthermore, in contrast to its inhibitory effects on norepinephrine-induced tachycardia in rat atria, milrinone enhanced the tachycardia produced by the adenylate cyclase activator forskolin. These results suggest that milrinone has complex actions on sympathetic control of heart rate and that β-adrenoceptor tachycardia occurs by mechanisms dependent on and independent of cyclic AMP.