Control of cardiac rate, contractility, and atrioventricular conduction by medullary raphe neurons in anesthetized rats.
The sympathetic actions of medullary raphé neurons on heart rate (HR), atrioventricular conduction, ventricular contractility, and rate of relaxation were examined in nine urethane-anesthetized (1-1.5 g/kg iv), artificially ventilated rats that had been adrenalectomized and given atropine methylnitrate (1 mg/kg iv). Mean arterial pressure (MAP), ECG, and left ventricular pressure were recorded. The peak rates of rise and fall in the first derivative of left ventricular (LV) pressure (dP/dtmax and dP/dtmin, respectively) and the stimulus-R ($-R) interval were measured during brief periods of atrial pacing at 8.5 Hz before and after ventral medullary raphé neurons were activated by dl-homocysteic acid (DLH, 0.1 M) or inhibited by GABA (0.3 M) in local microinjections (90 nl). LV dP/dtmax values were corrected for the confounding effect of MAP, determined at the end of the experiments after giving propranolol (1 mg/kg iv) to block sympathetic actions on the heart. DLH microinjections into the ventral medullary raphé region increased HR by 44 +/- 2 beats/min, LV dP/dtmax by 1,055 +/- 156 mmHg/s, and the negative value of LV dP/dtmin by 729 +/- 204 mmHg/s (all, P < 0.001) while shortening the $-R interval by 2.8 +/- 0.8 ms (P < 0.01). GABA microinjections caused no significant change in HR, LV dP/dtmax, or $-R interval but reduced LV dP/dtmin from -5,974 +/- 93 to -5,548 +/- 171 mmHg/s and MAP from 115 +/- 4 to 105 +/- 5 mmHg (both, P < 0.01). Rises in tail skin temperature confirmed that GABA injections effectively inhibited raphé neurons. When activated, the neurons in the ventral medullary raphé region thus enhance atrioventricular conduction, ventricular contractility, and relaxation in parallel with HR, but they provide little or no tonic sympathetic drive to the heart.
Salo
Lauren M
LM
Howard Florey Institute, University of Melbourne, Victoria, Australia.
Nalivaiko
Eugene
E
Anderson
Colin R
CR
McAllen
Robin M
RM
D000315:Adrenalectomy; D000320:Adrenergic Fibers / Q000187:drug effects / Q000502:physiology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000768:Anesthesia, General; D000818:Animals; D001283:Atrioventricular Node / Q000187:drug effects / Q000294:innervation*; D001286:Atropine Derivatives / Q000494:pharmacology; D001794:Blood Pressure; D004562:Electrocardiography; D006321:Heart / Q000187:drug effects / Q000294:innervation*; D006339:Heart Rate / Q000187:drug effects; D006710:Homocysteine / Q000008:administration & dosage / Q000031:analogs & derivatives; D008297:Male; D008845:Microinjections; D009200:Myocardial Contraction / Q000187:drug effects; D010276:Parasympatholytics / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011903:Raphe Nuclei / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D012881:Skin Temperature; D013564:Sympathetic Nervous System / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors; D016277:Ventricular Function, Left; D017725:Ventricular Pressure; D005680:gamma-Aminobutyric Acid / Q000008:administration & dosage
D000319:Adrenergic beta-Antagonists; D001286:Atropine Derivatives; D010276:Parasympatholytics; D006710:Homocysteine; C007956:homocysteic acid; D005680:gamma-Aminobutyric Acid; C006649:methylatropine; D011433:Propranolol