Cardiovascular responses to electrical stimulation of sympathetic nerves in the pithed mouse.

The pithed rat model has been used extensively to study peripheral cardiovascular responses to electrical stimulation of the sympathetic nervous system, as pithing eliminates central and reflex effects. However, since the transgenic mouse has become a standard and economical model organism, an electrically stimulated pithed mouse would facilitate a variety of studies. We have developed surgical techniques, drug doses and stimulation parameters for an electrically stimulated pithed mouse to study peripheral sympathetic nerve effects on blood pressure. Similar to the pithed rat, the pithed mouse showed voltage and frequency-dependent blood pressure responses to a pulsed train of electrical stimuli. In addition, alpha-adrenergic stimulation with phenylephrine gave a marked systolic pressor response, while the beta2 agonist salbutamol lowered diastolic blood pressure. Furthermore, pithed transgenic mice unable to synthesize catecholamines in adrenergic cells displayed smaller pressor responses than pithed control mice. In summary, the electrically stimulated pithed mouse can be used to study peripheral effects of the sympathetic system on cardiovascular dynamics unencumbered by central responses.

L-NAME raises systolic blood pressure in the pithed rat by a direct adrenal epinephrine releasing action.

It is generally thought that inhibition of nitric oxide synthase leads to blood pressure elevation largely through reduction in vascular levels of the vasodilator nitric oxide. However, there are several reports suggesting that NO synthase inhibitors cause adrenal epinephrine (E) release by both central and peripheral mechanisms. We investigated the role of adrenal E in the pressor effects of the nitric oxide synthase inhibitor L-NAME in the pithed rat to help distinguish central from peripherally mediated actions. L-NAME (10 mg/kg) raised both systolic and diastolic BP by about 30 mm Hg (P < .01) in the absence of exogenous electrical stimulation of sympathetic nerves. During stimulation at 10 V and frequencies of 1 or 2 Hz, systolic BP was about 70 mm Hg higher in L-NAME treated rats than in drug free stimulated rats. This enhancement of systolic BP by L-NAME was less pronounced at 5 or 10 Hz stimulation frequencies. Following these types of electrical stimulations of pithed rats, both plasma norepinephrine (NE) and E levels were dramatically elevated above resting plasma levels. L-NAME pretreatment of these electrically stimulated rats increased plasma E levels by an additional 60% and decreased NE by 18%. Acute adrenalectomy dramatically reduced plasma E levels and abolished the ability of L-NAME to enhance the pressor effect of sympathetic stimulation. In contrast, acute adrenalectomy of unstimulated pithed rats did not significantly reduce the pressor response to L-NAME. We conclude that adrenal E release may mediate much of the systolic pressor response of L-NAME in the stimulated pithed rat, but the magnitude of this effect varies with stimulation frequency. Since pithing disrupts central pathways, this induction of adrenal E release by L-NAME is a peripheral effect.

Selective peripheral regulation of noradrenaline and adrenaline release by nitric oxide.

1. Nitric oxide (NO) has complex effects on the sympathoadrenal and cardiovascular systems and may act at both central and peripheral loci. Nitric oxide appears to act directly on blood vessels and indirectly by modulating the sympathoadrenal system. In the present study, we investigated the contribution of catecholamine release from peripheral vascular and adrenal sympathetic nerves to the cardiovascular effects of the NO synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg). Our experiments were performed in pithed vagotomized rats to remove the influence of central and baroreflex pathways. 2. Spinal cord stimulations for 30 s periods at 1, 2, 5 and 10 Hz using pulses of 1 msec at 10 V caused marked increases in plasma adrenaline and noradrenaline. N(G)-Nitro-L-arginine methyl ester did not alter resting plasma catecholamine concentrations. However, L-NAME generally more than doubled stimulation-evoked release of adrenaline while reducing the extent of noradrenaline release relative to vehicle (saline)-treated controls. 3. N(G)-Nitro-L-arginine methyl ester significantly enhanced the vasopressor responses to spinal cord stimulation. The alpha1-adrenoceptor antagonist prazosin (0.2 mg/kg) reduced the pressor responses of electrically stimulated L-NAME-treated rats to levels below those of vehicle-treated control rats. 4. In the absence of electrical stimulation, L-NAME raised the blood pressure of pithed rats without altering plasma catecholamines and the pressor effect was briefly attenuated by L-arginine, but was unaffected by prazosin. 5. We conclude that the augmented pressor response to sympathetic stimulation in L-NAME-treated pithed rats is due largely to enhanced adrenal adrenaline release mediated by a peripheral mechanism. Stimulation of alpha(1)-adrenoceptors plays a major role in the pressor response to electrical stimulation of L-NAME-treated rats, but this is not due to L-NAME augmentation of noradrenaline release from vascular sympathetic nerves.

The pressor effect of NO synthase inhibition correlates to pre-existing systolic BP in the rat.

A number of studies have found that the vasopressor effect of nitric oxide (NO) synthase inhibition is small following treatment with hypotensive agents but is enhanced after hypertensive agents, and have implicated NO in the mechanism of action of these drugs. We investigated the hypothesis that the rate of vascular NO synthesis is directly related to blood pressure. The vasopressor effect of 10 mg/kg of L-nitro-L-arginine methyl ester (L-NAME) was studied in relation to changes in BP induced by a variety of treatments in both pentobarbital sodium anesthetized and pithed rats. BP reductions were induced by blood withdrawal, surgery and pithing. BP increases were made by injecting 10 and 15 microg/kg boluses of phenylephrine or by injecting 5% albumin solution. Pithing decreased baseline BP and attenuated the vasopressor effect of L-NAME while phenylephrine increased both BP levels and the hypertensive effect of L-NAME. Volume expansion with 5% albumin solution increased both BP and the vasopressor effect of L-NAME. Both surgery (abdominal incision) and withdrawal of 1 ml blood reduced BP and attenuated the pressor effect of L-NAME. When the results of all these studies were combined, systolic BP was found to correlate strongly with the vasopressor effect of L-NAME (R2 = 0.73, P < 0.0001). Diastolic BP correlated less well with L-NAME (R2 = 0.36, P < 0.0003). The results suggest that shear stress generated by blood flow during the systole releases NO, and lowers BP. The pressor effect of NO synthase inhibition is closely related to pre-existing systolic BP.