Relative roles of vagal and sympathetic effector mechanisms in the baroreflex control of myocardial contractility in conscious rabbits

Chronic central versus systemic blockade of AT(1) receptors and cardiac dysfunction in rats post-myocardial infarction

In rats, both central and systemic ANG II type 1 (AT(1)) receptor blockade attenuate sympathetic hyperactivity, but central blockade more effectively attenuates left ventricular (LV) dysfunction post-myocardial infarction (MI). In protocol I, we examined whether functional effects on cardiac load may play a role and different cardiac effects disappear after withdrawal of the blockade. Wistar rats were infused for 4 wk post-MI intracerebroventricularly (1 mg.kg(-1).day(-1)) or injected subcutaneously daily (100 mg x kg(-1) x day(-1)) with losartan. LV dimensions and function were assessed at 4 wk and at 6 wk post-MI, i.e., 2 wk after discontinuing treatments. At 4 and 6 wk post-MI, LV dimensions were increased and ejection fraction was decreased. Intracerebroventricular but not subcutaneous losartan significantly improved these parameters. At 6 wk, LV peak systolic pressure (LVPSP) and maximal or minimal first derivative of change in pressure over time (dP/dt(max/min)) were decreased and LV end-diastolic pressure (LVEDP) was increased. All four indexes were improved by previous intracerebroventricular losartan, whereas subcutaneous losartan improved LVEDP only. In protocol II, we evaluated effects of oral instead of subcutaneous administration of losartan for 4 wk post-MI. Losartan ( approximately 200 mg x kg(-1) x day(-1)) either via drinking water or by gavage similarly decreased AT(1) receptor binding densities in brain nuclei and improved LVEDP but further decreased LVPSP and dP/dt(max). These results indicate that effects on cardiac load by peripheral AT(1) receptor blockade or the pharmacokinetic profile of subcutaneous versus oral dosing do not contribute to the different cardiac effects of central versus systemic AT(1) receptor blockade post-MI.

Endothelin-1 produces heterogeneous regional haemodynamic effects in conscious rabbits

Blood flow in the renal artery, superior mesenteric artery and infra-renal abdominal aorta of conscious rabbits was measured by Doppler ultrasound. Arterial pressure, heart rate and blood flow responses were assessed following 0.2 and 0.8 nmol/kg intravenous endothelin-1. The effects of the following antagonists on these responses were examined: phentolamine, propranolol, scopolamine, captopril, nifedipine, indomethacin, the V1-vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP and the competitive nitric oxide (NO) synthase inhibitor NG-nitro L-arginine (NOLA). Hindlimb resistance and arterial pressure responded in two phases, initial vasodilatation followed by vasoconstriction. Renal and mesenteric vasoconstriction occurred without initial vasodilatation. Following 0.2 nmol/kg endothelin-1, arterial pressure decreased by 18.5 +/- 0.8 mmHg, then increased by 25.2 +/- 1.7 mmHg (n = 27). Heart rate changed reciprocally. Renal resistance increased by 533 +/- 73% (n = 12). Mesenteric resistance increased by 420 +/- 34%. Hindlimb resistance decreased 54 +/- 2% (n = 12, all P < 0.01) then increased slightly (P < 0.05). All changes were greater at 0.8 nmol/kg, particularly the hindlimb vasoconstriction. The only antagonist to alter significantly these responses was NOLA, which in the hindlimb attenuated the vasodilatation and accentuated the vasoconstriction. We conclude that most of the haemodynamic effects of endothelin-1 are direct, but that NO generated by NO synthase causes part of the hindlimb vasodilatation, and that endothelin-1-induced vasoconstriction is attenuated by release of NO.

Cardiac dysrhythmias complicating maxillofacial surgery

Cardiac asystole, bradycardia and other dysrhythmias are complications associated with surgery of the craniofacial and maxillofacial structures. These dysrhythmias have been discussed frequently in ophthalmology but less attention has been paid to this problem in the maxillofacial surgery literature. Eight patients, 6 men and 2 women, who underwent maxillofacial surgery during a 16-month period at the Victoria General Hospital, Halifax, Canada demonstrated either asystole or bradycardia. This group of patients represents 1.6% of the total number of patients treated during the 16-month period. Six patients demonstrated either asystole or bradycardia during stretching of the soft tissues associated with advancement of the maxilla after bony mobilization. Two patients had either asystole or bradycardia during manipulation of the temporalis muscle fibres during surgery to correct total bony ankylosis of the temporomandibular joint. The etiology of this relatively rare but troublesome complication is discussed.

Effects of neuropeptide Y on baroreflex control of heart rate and myocardial contractility in conscious rabbits

1. The effects of intravenous (i.v.) neuropeptide Y (NPY, 10 micrograms/kg bolus) on the stimulus-response curves relating changes in heart period (HP) and in peak left ventricular (LV) dP/dt to acute changes in mean arterial pressure (MAP) were determined in conscious, normotensive rabbits. 2. The relationship between increases and decreases in MAP and the subsequent changes in HP were represented by a sigmoid-shaped curve described by a logistic function. Following NPY administration there was a baroreflex-dependent increase in the maximum slope (sensitivity) at the midpoint of this MAP-HP curve from 7.0 +/- 0.5 to 10.6 +/- 1.3 ms/mmHg (P less than 0.05). NPY caused an upward shift in the whole curve which reflected the NPY-induced bradycardia and was independent of baroreflexes. 3. The relationship between increases in MAP and decreases in peak LV dP/dt was determined during fixed-rate atrial pacing to prevent the effects of the accompanying bradycardia. Increases in MAP and the corresponding reductions in peak LV dP/dt were represented by an exponential function. The slope of the curve, measured at its origin 5-15 min after NPY administration, was reduced from -0.9 +/- 0.2 to -0.4 +/- 0.1 units (P less than 0.05). 4. The effects of NPY are consistent with an action on efferent connections of the arterial baroreceptor reflex, mediated through a reduction in cardiac beta-adrenergic tone. They would also be explained through actions on the afferent or central neural connections of the baroreflex.

Progressive attenuation of the carotid baroreflex control of blood pressure and heart rate during exercise

With the use of a variable pressure neck chamber, the effect of supine bicycle exercise on the carotid baroreceptor reflex was investigated in 10 normal volunteers who were studied at rest and during moderate and severe exercise, i.e., at 33% and 66% of their previously determined maximal exercise capacity. Deactivation of the carotid baroreceptors by a decrease in carotid transmural pressure produced a greater (p less than 0.001) increase in mean intra-arterial pressure at rest (+11.2 +/- 0.2 [SE] mm Hg) than during moderate (+9.0 +/- 0.2 mm Hg) and severe (+6.1 +/- 0.1 mm Hg) exercise. Heart interval decreased less (p less than 0.005) at rest (-73 +/- 3.2 msec) than during exercise (-0.7 +/- 0.7 msec and -4.6 +/- 0.2 msec, respectively). Furthermore, activation of the carotid baroreceptors by an increase in carotid transmural pressure provoked a greater (p less than 0.005) reduction in mean intra-arterial pressure at rest (-11.5 +/- 0.2 mm Hg) than during exercise (-9.8 +/- 0.1 mm Hg and -7.5 +/- 0.1 mm Hg, respectively), while the lengthening of heart interval was also more (p less than 0.005) pronounced at rest (+97 +/- 4.3 msec) than during moderate (+44 +/- 1.2 msec) and severe (+7.6 +/- 0.3 msec) exercise. Finally, carotid baroreceptor sensitivity was defined as the slope of the changes in blood pressure and heart interval that occurred with increasing (baroreceptor deactivation) and decreasing (activation) neck tissue pressure. On average, baroreceptor sensitivity was consistently smaller (p less than 0.05) during exercise, as compared to rest.(ABSTRACT TRUNCATED AT 250 WORDS)