Chronic
hypoxia induces an overall sympathetic hyperactivation associated with a myocardial beta-receptor desensitization. The mechanisms involved in this desensitization were evaluated in 32 male Wistar rats kept in a hypobaric pressure chamber (PO2 = 40 Torr, atmospheric pressure = 450 Torr) for 5 days. In hypoxic compared with normoxic conditions, plasma
norepinephrine (NE) levels were higher (2.1 +/- 0.7 vs. 0.6 +/- 0.2 ng/ml) with no difference in the plasma
epinephrine levels (2.2 +/- 0.7 vs. 1.8 +/- 0.3 ng/ml). In
hypoxia neuronal NE uptake measured by [3H]NE was decreased by 32% in the right ventricle (RV) and by 35% in the left ventricle (LV), and [3H]
mazindol in vitro binding showed a decrease in uptake-1
carrier protein density by 38% in the RV and by 41% in the LV. In vitro binding assays with [3H]
CGP-12177 indicate beta-
adrenoceptor density reduced by 40% in the RV and by 32% in the LV, and this was due to reduced beta1-subtype fraction (competition binding experiments with
practolol).
Hypoxia reduced the production of cAMP induced by
isoproterenol (36% decrease in the RV and 41% decrease in the LV), 5'-guanylylimododiphosphate (40% decrease in the RV and 42% decrease in the LV), and
forskolin (39% decrease in the RV and 41% decrease in the LV) but did not alter the effect of
MnCl2 and NaF. Quantitation of inhibitory
G-protein alpha-subunit by immunochemical analysis showed a 46% increase in the cardiac-specific
isoform Gialpha2 in hypoxic hearts. The present data demonstrate that in rats 5-day
hypoxia leads to changes in pre- and postsynaptic myocardial
adrenergic function. The myocardial desensitization associated with both a reduction in externalized beta1-adrenoceptor and an increase in inhibitory
G-protein subunit may be caused by increased synaptic NE levels due to impaired uptake-1 system.