Presynaptic regulation of cardiac sympathetic function in guinea pigs

Cold stress (4 degrees C) induces a pressor response and variable increases in an index of sympathetic neural function, the rate constant of norepinephrine turnover, kNE. In heart, presynaptic cholinergic muscarinic and alpha-2 adrenergic influences may contribute to regional variation in responses of kNE to cold stress. Animals were pretreated with vehicle, a muscarinic cholinergic antagonist, quinuclidinyl benzilate (QNB), an alpha-2 adrenergic antagonist, yohimbine (YOH) or combined QNB + YOH. An increase in kNE was determined from incorporation of radiolabeled tyrosine into norepinephrine in a control period at 24 degrees C and again at 4 degrees C. The increment in kNE factored by the increment in blood pressure indicated the extent of increased sympathetic function in each cardiac region. In sino-atrial node, sympathetic function was increased significantly (P less than .05) by QNB + YOH compared to other treatments, suggesting that both cholinergic and alpha-2 adrenergic presynaptic influences were important. In contrast, in right and left ventricles, YOH or QNB + YOH, but not QNB alone, increased sympathetic function significantly, suggesting that only alpha-2 adrenergic influences were important. These data support the concept that presynaptic regulation of cardiac sympathetic function differs in sino-atrial node and ventricles of guinea pigs during activation with cold stress.