Autonomic contribution to the blood pressure and heart rate variability changes in early experimental hyperthyroidism.
OBJECTIVE
To study the interaction between autonomic nervous activity and thyroid hormones in the control of heart rate (HR) and blood pressure (BP).


METHODS
Thyrotoxicosis was produced by injections of L-thyroxine (0.5 mg/kg/day for five days). Blockers were atropine (0.5 mg/kg), atenolol (1 mg/kg) or prazosin (1 mg/kg). Eight animals were studied in each group. Spectral analyses was performed using continuous BP time series obtained in conscious rats.


RESULTS
Thyroxine treatment was sufficient to induce a significant degree of tachycardia (423+/-6 vs 353+/-4 bpm, P < 0.001, unpaired Student's t test), systolic BP elevation (142+/-3 vs 127+/-2 mmHg, P < 0.001) and cardiac hypertrophy (1.165+/-0.017 vs 1.006+/-0.012 g, P < 0.001). The intrinsic HR was markedly increased after treatment with thyroxine (497+/-16 vs 373+/-10 bpm, P < 0.05). Vagal tone was positively linearly related to intrinsic HR (r = 0.84, P< 0.01). Atenolol neither modified HR nor BP variability in rats with hyperthyroidism. The thyrotoxicosis was associated with a reduction of the 0.4 Hz component of BP variability (modulus 1.10+/-0.07 vs 1.41+/-0.06 mmHg, P < 0.01). Prazosin was without effect on this 0.4 Hz component in hyperthyroid animals.


CONCLUSIONS
These data show a functional diminution of the vascular and cardiac sympathetic tone in early experimental hyperthyroidism. The marked rise in the intrinsic HR could be the main determinant of tachycardia. The BP elevation may reflexly induce vagal activation and sympathetic (vascular and cardiac) inhibition.
Safa-Tisseront
V
V
Laboratoire de Pharmacologie, CNRS URA 1482, Faculté de Médecine Necker, Paris, France.
Ponchon
P
P
Laude
D
D
Elghozi
J L
JL
D000818:Animals; D001262:Atenolol / Q000494:pharmacology; D001285:Atropine / Q000494:pharmacology; D001341:Autonomic Nervous System / Q000187:drug effects / Q000503:physiopathology*; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology*; D006332:Cardiomegaly / Q000139:chemically induced / Q000503:physiopathology; D004195:Disease Models, Animal; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology*; D006980:Hyperthyroidism / Q000209:etiology / Q000503:physiopathology*; D008297:Male; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013610:Tachycardia / Q000209:etiology / Q000503:physiopathology; D013963:Thyroid Hormones / Q000502:physiology; D013974:Thyroxine / Q000008:administration & dosage; D014630:Vagus Nerve / Q000187:drug effects / Q000503:physiopathology
D013963:Thyroid Hormones; D001262:Atenolol; D001285:Atropine; D013974:Thyroxine; D011224:Prazosin