Noradrenergic transmission in the isolated portal vein of the spontaneously hypertensive rat.

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Abstract
The effect of electrical field stimulation (1, 2, 5, 0 Hz for a total of 480 pulses at 15 min intervals) on the release of 3H-norepinephrine from the superfused portal vein of spontaneously hypertensive rats (SHR) or Wistar-Kyoto rats (WKY) of various ages was studied. The ages of the animals were (in weeks) 5-6 (prehypertensive), 8-10 (young hypertensives), 16-18 (older hypertensives) and 28 (mature hypertensives). There was no difference in the release of 3H-norepinephrine or developed tension of the portal vein to any frequency of field stimulation of SHR or WKY at 5-6 wk of age. However, there was a significantly greater release of 3H-norepinephrine and developed tension of veins of SHR in response to low (1 or 2 Hz) but not high frequencies (5 or 10 Hz) at 8-10, 16-18 and 28 wk of age. Vessels from hypertensive animals also developed greater resting tension and spontaneous activity, which was reduced to that of WKY in the presence of an .alpha.-adrenergic antagonist. The .alpha.2 selective adrenergic antagonist yohimbine produced the same degree of enhancement of release of 3H-norepinephrine to field stimulation of veins obtained from both SHR and WKY at 5-6, 8-10 and 16-18 wk of age. However, the facilitory effect of yohimbine was significantly attenuated in portal veins obtained from SHR at 28 wk of age compared to age-matched WKY. The .beta.-adrenergic agonist isoproterenol produced a similar degree of enhancement of the field-stimulation-induced release of 3H-norepinephrine from vessels obtained from SHR and WKY at all ages. In contrast, the facilitory effect of angiotensin was significantly enhanced in vessels obtained from 10-, 12- and 28-wk-old SHR compared to age-matched WKY. Results demonstrate alterations in the in vitro evoked release of norepinephrine from blood vessels obtained form hypertensive animals. In additions, alterations were demonstrated in the functional activity of presynaptic receptors in peripheral noradrenergic neurons innverating the portal vein in hypertensive animals compared to age-matched controls. These alterations may be important in the development and maintenance of hypertension in the SHR.