The k-complex vasoconstrictor response: evidence for central vasomotor downregulation in borderline hypertension

Central sympathetic outflow and the baroreflex were measured as an integrated cardiopressor response to a noise stimulus during normal sleep, in 13 normotensives and 14 borderline hypertensives. The pressor response latency was prolonged in the borderline hypertensives, and the stimulus-induced increase in systolic and diastolic intra-arterial pressure was greater in the borderline hypertensives (23.0 +/- 7.3-16.2 +/- 5.6 mmHg) compared with the normotensives (16.7 +/- 5.9-13.6 +/- 4.6 mmHg; P less than 0.001), but the heart rate was not increased. Despite the increased pressor response, the baroreflex-mediated fall in the heart rate following the pressor response was reduced in the borderline hypertensives, indicating secondary adaptation to hypertension and a resetting, or downregulation, of central vasomotor control mechanisms in the borderline hypertensives relative to normotensives.