Nifedipine potentiates cardiopulmonary baroreflex control of sympathetic nerve activity in healthy humans: direct evidence from microneurographic studies

Nifedipine augments baroreflex mechanisms in in vivo animal models. Previous studies in our laboratory demonstrate that nifedipine potentiates baroreflex control of heart rate and vascular resistance in normal human subjects. To further define the neuroeffector mechanism of the autonomic effects of nifedipine, we directly measured postganglionic sympathetic nerve activity to muscle (MSNA, microneurography), before and after drug administration, during selective unloading of cardiopulmonary baroreceptors with lower body negative pressure (-10 mm Hg, LBNP-10), and during the cold pressor test. Twenty-three normal subjects (age, 23 +/- 1 years; mean +/- SEM) were studied in the control state and 20 minutes after administration of either nifedipine (10 mg s.l., 10 subjects), during nitroprusside infusion (0.37 +/- 0.03 microgram/kg/min i.v., eight subjects), or 20 minutes after sublingual administration of placebo (five subjects). We measured systemic arterial pressure, central venous pressure, heart rate, and MSNA. Nifedipine and nitroprusside produced similar increases in resting heart rate and MSNA and similar decreases in central venous pressure, whereas placebo had no effect on resting hemodynamics. During LBNP-10, hemodynamic changes were not significantly different among the three treatment groups. However, the percentage increase in MSNA during LBNP-10 was significantly augmented from a 24 +/- 9% increase before nifedipine to a 56 +/- 7% increase after nifedipine (p less than 0.05). Decreases in central venous pressure with LBNP-10 were nearly identical before compared with after nifedipine. Thus, nifedipine increased the cardiopulmonary baroreflex sympathetic sensitivity (change in total MSNA per mm Hg decrease in central venous pressure during LBNP-10) from 26.5 +/- 10.7 units/mm Hg to 74.9 +/- 19.0 units/mm Hg (p less than 0.01). In contrast, administration of hemodynamically similar doses of nitroprusside resulted in an attenuation of MSNA responses to LBNP-10. During LBNP-10, MSNA increased 57 +/- 12% before nitroprusside but only 14 +/- 4% during nitroprusside (p less than 0.01). The cardiopulmonary baroreflex sympathetic sensitivity was not significantly altered by nitroprusside (45.1 +/- 12.4 units/mm Hg before compared with 33.1 +/- 20.8 units/mm Hg during nitroprusside, p = NS). Placebo had no effect on the responses to LBNP-10. Nifedipine did not augment MSNA responses to the cold pressor test. To evaluate the linearity of sympathetic respon