Genetic differences have been demonstrated in cardiovascular responsiveness, but these responses have not correlated with autonomic nervous system activation as measured by catecholamine excretion. If genetic factors are determinants of the cardiovascular responses, a relationship should exist between twins; if biochemical responses primarily indicate arousal, their concordance might be less likely.Twelve pairs each of monozygotic and dizygotic twins were studied. Blood pressure, pulse rate, catecholamine excretion, and free fatty acid (FFA) levels were measured before and after administration of two stimuli-a frustrating mental exercise (the Stroop color test) and ischemic pain to the arm.Both stimuli caused brisk cardiovascular responses; the response to ischemic pain was greater than that to color for blood pressure, but not for pulse rate. In both groups, norepinephrine excretion fell, but epinephrine and vanilmandelic acid (VMA) rose, and FFA increased slightly. Significant correlations were noted in blood pressure and pulse rate measurements between individual twins, but were absent in the biochemical measurements.Using a technique described to distinguish between genetic and environmental variability, intrapair variance (between twins) was less than interpair variance (between pairs) for most cardiovascular measures, but not for catecholamine excretion. Intrapair variance for the cardiovascular responses did not change with stimulation, but increased for catecholamines. Dizygote variances were slightly greater than in monozygotes for the cardiovascular measurements, but not for the catecholamine excretion.The results confirm the lack of a direct association between cardiovascular changes and autonomic nervous system activation. Furthermore, they suggest that genetic factors play a greater role in controlling cardiovascular responsiveness than the adrenergic biochemical correlates of this responsiveness; the latter may be affected primarily by the test environment and anticipatory anxiety.A N A SERIES OF STUDIES we have at-systematic fashion, with the goal of detempted to describe blood pressure re-termining the quantitative importance sponsivity to psychological stimuli in a of this factor in hypertensive disease. 1

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