Loss of maximum attenuation and receptor reserve for isoprenaline at the beta 2-adrenoceptors of the portal veins of hypertensive rats.
OBJECTIVE
To test the hypothesis that vascular beta 2-adrenoceptor hyporesponsiveness in spontaneously hypertensive rats (SHR) is not induced by increased blood pressure or venous hypertrophy.


METHODS
We compared the attenuating effects or isoprenaline, sodium nitroprusside and verapamilon the portal veins from Wistar-Kyoto (WKY) rats and SHR. studied the effects of slowly reversible beta-adrenoceptor antagonists, bromoacetylalprenololmenthane (BAAM) and ICI 147798, on the isoprenaline responses in order to determine the affinity and fractional beta 2-adrenoceptor occupancy-response relationships for isoprenaline.


RESULTS
The SHR portal veins did not develop hypertrophy. There was a small reduction in the sensitivity to isoprenaline and a marked reduction in the maximum attenuation of hypertension caused by isoprenaline. The sensitivity and efficacy of sodium nitroprusside and verapamil were not altered by hypertension. BAAM and ICI 147798 inhibited the isoprenaline responses and reduced the maximum attenuation to isoprenaline. In the WKY rat portal vein the dissociation constant (KA) values for isoprenaline were independent of BAAM concentration, and was 1.78 +/- 0.32 x 10(-7) mol/l. Similar isoprenaline KA values were obtained from the ICI 147798 data and in the SHR portal vein. In the WKY rat portal vein, from the BAAM data, it was calculated that isoprenaline produced 50, 95 and 100% maximum responses by occupying 6 +/- 1, 20 +/- 3 and 43 +/- 5% (n = 21) of the available beta 2-adrenoceptors, respectively. Similar occupancy-response relationships were obtained in the WKY rat portal vein from the ICI 147798 data. At each level of isoprenaline response the receptor reserve was significantly smaller in the SHR than it was in the WKY rat portal vein. Thus, from the BAAM data, isoprenaline produced 50, 95 and 100% maximum responses by occupying 14 +/- 3, 33 +/- 6 and 58 +/- 7% (n = 15) of the available SHR portal vein beta 2-adrenoceptors, respectively.


CONCLUSIONS
The SHR portal vein displays a selective beta 2-adrenoceptor hyporesponsiveness in the absence of a raised blood pressure or hypertrophy. This beta 2-adrenoceptor-associated hyporesponsiveness consisted of a marked loss of maximum attenuation in response to isoprenaline and of beta 2-adrenoceptor reserve for isoprenaline responses.
Doggrell
S A
SA
Department of Pharmacology and Clinical Pharmacology, School of Medicine, University of Auckland, New Zealand.
Surman
A J
AJ
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000526:Alprenolol / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D006973:Hypertension / Q000188:drug therapy / Q000503:physiopathology*; D006984:Hypertrophy; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D009599:Nitroprusside / Q000494:pharmacology; D011169:Portal Vein / Q000187:drug effects*; D011719:Pyrazines / Q000494:pharmacology; D051381:Rats; D011918:Rats, Inbred SHR; D011921:Rats, Inbred WKY; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D014665:Vasodilator Agents / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D011719:Pyrazines; D011943:Receptors, Adrenergic, beta; D014665:Vasodilator Agents; D009599:Nitroprusside; C045331:bromoacetylalprenololmenthane; C053707:ICI 147798; D000526:Alprenolol; D014700:Verapamil; D007545:Isoproterenol