Nitric oxide enhances the inotropic response to beta-adrenergic stimulation in the isolated guinea-pig heart.
Nitric oxide (NO) exerts several effects on myocardial contraction, including enhancement of relaxation and diastolic function, modulation of beta-adrenergic inotropic responses, and inotropic effects in the absence of agonist pre-stimulation. Different effects have been observed in different species and preparations, and it is unclear whether they are species- or preparation-specific, or whether they represent a range of responses that can manifest in most mammalian species. We therefore examined the effects of NO on the inotropic response to beta-adrenergic stimulation in the isolated guinea-pig heart, a species in which we have previously shown that NO enhances basal left ventricular (LV) relaxation and modulates the Frank-Starling response. Isolated ejecting hearts were perfused with Krebs buffer at constant placed heart rate (1 microM) indomethacin, 37 degrees C, constant loading conditions), and high fidelity LV pressure was monitored by an apical 2 F Millar catheter. All hearts were initially treated with dobutamine (0.1 microM) and then, once the peak inotropic and chronotropic response had been established, with either (a) no further treatment (n = 6), (b) the NO donor sodium nitroprusside (1 microM, n = 6; 10 microM, n = 6), or (c) the specific agonist for NO release, substance P (0.1 microM, n = 6). Dobutamine (0.1 microM) produced a rapid positive inotropic and chronotropic response, associated with a fall in LV end-diastolic pressure (LVEDP) and a rise in coronary flow. The positive inotropic effect of dobutamine declined over 20-28 minutes, while the chronotropic response persisted over this period. Low dose sodium nitroprusside (1 microM) delayed the decline in the inotropic response to dobutamine and exaggerated the fall in LVEDP. Similar effects were observed with substance P (0.1 microM). In contrast, a higher dose of sodium nitroprusside (10 microM) did not alter the response to dobutamine. These data indicate that "low dose" NO augments the inotropic response to beta-adrenergic stimulation in the isolated ejecting guinea-pig heart, in addition to its previously reported effects on basal LV relaxation in the same preparation.
Department of Cardiology, University of Wales College of Medicine, Cardiff, United Kingdom.
B D
BD
Prendergast
P
P
MacCarthy
J F
JF
Wilson
A M
AM
Shah
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002302:Cardiac Output / Q000187:drug effects; D004280:Dobutamine / Q000494:pharmacology; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D009569:Nitric Oxide / Q000494:pharmacology*; D009599:Nitroprusside / Q000494:pharmacology; D013373:Substance P / Q000494:pharmacology; D014665:Vasodilator Agents / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D014665:Vasodilator Agents; D009599:Nitroprusside; D009569:Nitric Oxide; D013373:Substance P; D004280:Dobutamine