Presynaptic regulation of cardiac sympathetic function in hypoxic guinea pigs

Intermittent hypoxia modulates nNOS expression and heart rate response to sympathetic nerve stimulation

Nitric oxide (NO) decreases norepinephrine (NE) release and the heart rate (HR) response to sympathetic nerve stimulation (SNS). We tested the hypothesis that the enhanced HR response to sympathetic activation following chronic intermittent hypoxia (IH) results from a peripheral modulation of pacemaking by NO. Isolated guinea pig double atrial/right stellate ganglion preparations were studied from animals that had been exposed to IH (n = 20) and control animals (n = 22). The HR response to SNS was significantly enhanced in the IH group compared with the controls. However, the increase in HR with cumulative doses (0.1--10 microM) of bath-applied NE was similar in both groups. Western blot analysis showed less neuronal NO synthase in the right atria from the IH group. In IH animals, the NO synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA; 100 microM) did not alter the increased HR response to SNS, whereas in control animals L-NNA significantly increased the HR response to SNS; an effect that was reversed with excess L-arginine. In conclusion, the enhanced HR response to SNS after IH may be related to a decreased inhibitory action of NO on presynaptic NE release.

Hypoxia- and normoxia-induced reversibility of autonomic control in Andean guinea pig heart

We herein describe the regulation of cardiac receptors in a typical high-altitude native animal. Heart rate response to isoproterenol (HRIso) (beats.min-1.mg Iso.kg-1) and atropine, the density of beta-adrenergic (beta AR) and muscarinic (M2) receptors, and the ventricular content of norepinephrine (NE) and dopamine (DA) were studied in guinea pigs (Cavia porcellus). Animals native to Lima, Peru (150 m) were studied at sea level (SL) and after 5 wk at 4,300-m altitude (SL-HA). Animals native to Rancas [Pasco, Peru (4,300 m)] were studied at high altitude (HA) and after 5 wk at SL (HA-SL). HA animals had a lower HRIso, maximum number of beta AR binding sites (Bmax), beta AR dissociation constant (Kd), NE, and DA (P < 0.05) and a higher M2 Bmax (P < 0.001) when compared with the SL group. HA-SL showed an increase of the HRIso, beta Ar Kd, and NE (P < 0.05) and a decrease of the M2 Bmax and Kd (P < 0.0001) when compared with the HA group. The present study demonstrates the differential regulation and reversibility of the autonomic control in the guinea pig heart.

Sympathetic denervation blocks blood pressure elevation in episodic hypoxia

We have previously described a rat model that responds to repetitive episodic hypoxia (FiO2 nadir 3-5% for 12 seconds every 30 seconds for 7 hr/day for 35 days) with chronic increase in arterial blood pressure. The purpose of the current study was to determine if peripheral sympathetic nervous system denervation blocks this persistent blood pressure elevation. Chemical sympathetic denervation was achieved and maintained by three intraperitoneal injections (100 mg/kg 6-hydroxydopamine) on days 1, 3, and 27 of a 47-day experiment in two groups of rats. One denervated group was subjected to episodic hypoxia for 40 consecutive days beginning on day 7 and the other remained unhandled in their usual cages. A third group was injected with vehicle only and subjected to the same episodic hypoxia while a fourth group remained unhandled for 40 days. The vehicle-treated, episodic hypoxia-exposed group showed a 7.7 mm Hg increase in mean arterial blood pressure (conscious, unrestrained) over the 40-day period, whereas all other groups showed a decrease in mean arterial pressure. The left ventricle and septum/whole body weight ratio was higher in both episodic hypoxia-exposed groups at the end of the study. Plasma epinephrine in both groups administered 6-hydroxydopamine was higher on day 6 than in the vehicle-injected rats. Measurement of catecholamines in cardiac muscle homogenate confirmed denervation in 6-hydroxydopamine animals. These results indicate that the peripheral sympathetic nervous system is necessary for the persistent increase in blood pressure in response to repetitive episodic hypoxia.