Adrenergic coronary vasoconstriction in acute right ventricular hypertension

This study tested the hypothesis that alpha-adrenergically mediated coronary vasoconstriction during the haemodynamic stress of right ventricular hypertension antagonises the effect of adenosine. Myocardial blood flow and plasma catecholamines were measured in 12 awake, but mildly sedated, chronically instrumented dogs during acute pulmonary artery constriction (PAC) before and during infusion of adenosine (1 mg.kg^-1 iv), with and without prior alpha blockade with phenoxybenzamine (10 mg/kg iv). PAC was performed by gradual inflation of a pulmonary artery hydraulic occluder until aortic pressure began to fall, after which aortic pressure was restored to control levels by aortic constriction. Heart rate was held constant by pacing. Two distinct patterns of response to adenosine during PAC were observed. Six dogs (group A) demonstrated attenuation of adenosine induced coronary vasodilatation; left ventricular (LV) myocardial flow fell from 7.35 ± 1.14 ml.min^-1.g^-1 (±SE) during adenosine to 3.63 ± 0.81 during PAC and adenosine (p < 0.05). In the six remaining dogs (group B) attenuation was not observed (LF flow = 6.51 ± 0.92 during adenosine; 8.14 ± 1.11 during PAC and adenosine). During PAC, both the severity of RV hypertension and levels of circulating catecholamines were greater in group A than in group B. Alpha-blockade partially restored adenosine responsiveness in group A during PAC, and LV flow increased to 6.23 ± 0.80 (p < 0.05). We propose that attenuation of the coronary flow response to adenosine infusion during PAC is related to the severity of PAC and alpha-adrenergically mediated coronary vasoconstriction.