Contrasting effects of phentolamine and nitroprusside on neural and cardiovascular variability.
The relative contributions of a central neural oscillator and of the delay in alpha-adrenergic transmission within the baroreflex loop in the predominance of low-frequency (LF) cardiovascular variability during sympathetic activation in humans are unclear. We measured R-R interval (RR), muscle sympathetic nerve activity (MSNA), blood pressure (BP), and their variability in 10 normal subjects during sympathetic activation achieved by BP lowering with sodium nitroprusside (SNP) and alpha-adrenergic blockade using phentolamine. SNP and phentolamine induced comparable reductions in BP (P > 0.25). Despite tachycardia and sympathetic activation with both SNP and phentolamine, LF variability in RR, MSNA, and BP increased during SNP and decreased during phentolamine (SNP: RR +20 +/- 6%, MSNA +3 +/- 5%, systolic BP +9 +/- 6%, diastolic BP +7 +/- 5%; phentolamine: RR -2 +/- 7%, MSNA -34 +/- 6%, systolic BP -16 +/- 8%, diastolic BP -13 +/- 4%, P < 0.05 except systolic BP, where P = 0.09). Thus LF variability is reduced when sympathetic activation is induced by alpha-adrenergic blockade. This suggests that alpha-adrenergic transmission within the baroreflex loop may contribute importantly to the predominance of LF cardiovascular variability associated with sympathetic excitation in humans.
van de Borne
P
P
Department of Cardiology/Hypertension Clinic, Erasme Hospital, 1070 Brussels, Belgium.
Rahnama
M
M
Mezzetti
S
S
Montano
N
N
Porta
A
A
Degaute
J P
JP
Somers
V K
VK
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000328:Adult; D017704:Baroreflex / Q000187:drug effects; D004562:Electrocardiography; D006321:Heart / Q000187:drug effects* / Q000294:innervation / Q000502:physiology; D006801:Humans; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology; D009599:Nitroprusside / Q000494:pharmacology*; D010646:Phentolamine / Q000494:pharmacology*; D013564:Sympathetic Nervous System / Q000187:drug effects* / Q000502:physiology; D014665:Vasodilator Agents / Q000494:pharmacology*
D000317:Adrenergic alpha-Antagonists; D014665:Vasodilator Agents; D009599:Nitroprusside; D010646:Phentolamine