The aim of this study was to evaluate the role of endogenous histamine in the regulation of reactive hyperaemia (RH) and coronary autoregulation in isolated rat hearts. The basal release of cardiac histamine (perfusion pressure 60 cm H2O) amounted to 100-200 pmol/min/g wL During the first 15 s following 30 s of coronary occlusion, the release of histamine increased about three times and returned to basal levels after approximately 90 s, paralleling the changes of coronary flow (CF). Blockade of Hi-receptors increased basal CF by 23±2 %, significantly reduced blood flow debt and prolonged the duration of RH. Blockade of H2- and H3-receptors produced a significant decline of CF, decreased RH flow and diminished RH by 40±3 %. Blockade of all three classes of histamine receptors indicated that endogenous histamine exerts predominantly vasodilatory effects (mediated by H2- and H3-receptors) on coronary circulation. Histamine-induced vasodilation appears to be NO-dependenL Changes of coronary perfusion pressure from 20 to 120 cm H2O were accompanied by an almost linear decrease of histamine release from about 200 to 40-50 pmol/min/g wL Blockade of histamine receptors decreased, while L-NAME significantly widened the autoregulatory range of the isolated rat heart, reduced CF and release of NO, but reversed the pattern of histamine release leaving the autoregulatory range unaltered, which indicate that endogenous histamine does not play a role in the regulation of coronary autoregulation.
Nitric oxide plays an important role in the control of basal coronary tone and mediation of reactive hyperaemic flow response following short-term coronary occlusion. The results presented in this report indicate that NO is involved in the modulation of coronary autoregulation in isolated rat hearts. Isolated rat hearts exhibit autoregulation of coronary flow (CF) between 50 and 80 cm H2O of coronary perfusion pressure (CPP). Within this autoregulatory range NO release (measured as nitrite) varies from
1.7±0.3 to 2.2±0.7 nmol/min/g wL Below the autoregulatory range it decreases slightly, while above this there is more than a twofold increase. Changes of NO release are accompanied by directly proportional changes of cGMP release. The release of hypoxanthine + xanthine shows a reciprocal relationship to CF values. The inhibition of NO synthesis showed a reciprocal relationship with CF values. Inhibition of NO synthesis by L-NAME (30 /¿mol/l) significantly reduces CF over the entire range of CPP changes (20-120 cm H2O), but much less at lower than at higher pressure values. Therefore, the autoregulatory range is significantly widened to CPP of 40-100 cm H2O. Theophylline (30 yumol/l) reduces CF by 15-25 % throughout the entire range of CPP changes. Hence, the CPP-CF curve is shifted downwards without significant changes of the autoregulatory range. Theophylline-induced reduction of NO release is CPP-dependent: as greater as CPP lower. When L-NAME is coadministered with theophylline, CF is additionally reduced while widened autoregulatory range is shifted to the right