Chronic steroid contraceptive treatment decreases renal alpha-adrenoceptor levels in the rat

Effect of contraceptive steroid and enalapril treatment of systolic blood pressure and plasma renin-angiotensin in the rat

Previous work has demonstrated contraceptive steroid-induced hypertension in rats. Here, we examined the relationship between steroid-induced hypertension and components of the renin-angiotensin system. Female Sprague-Dawley rats were injected s.c. with 0.2 micrograms ethynyloestradiol, 2.0 micrograms levonorgestrel, a combination of both or vehicle, six days per week. A second group of rats received 2.0 micrograms enalapril maleate, enalapril plus ethynyloestradiol or levonorgestrel, or vehicle. Systolic blood pressure increased with both ethynyloestradiol. (6 weeks, +17 mmHg; 12 weeks, +32 mmHg) and levonorgestrel (6 weeks, +24 mmHg) treatment. This effect of levonorgestrel was attenuated by co-administration of enalapril, which also reversed the hypertension seen with ethynyloestradiol. Ethynyloestradiol, but not levonorgestrel treatment caused a significant increase in plasma renin concentration, plasma renin activity, and plasma angiotensin II at both 6 and 12 weeks. Plasma renin substrate was increased by ethynyloestradiol at 3, 6 and 12 weeks, prior to the observed increase in systolic blood pressure. Combined steroid treatment had less pronounced effects. Enalapril alone or in combination with ethynyloestradiol decreased plasma renin concentration, activity and angiotensin II, and in combination with levonorgestrel decreased plasma renin concentration, substrate and activity (6 weeks only) but not angiotensin II. The data indicate a positive relationship between hypertension and the renin-angiotensin system with ethynyloestradiol, but not levonorgestrel treatment in rats.

Renal and myocardial adrenoceptors in steroid contraceptive-induced hypertension in rats

1. Systolic blood pressure (SBP), bodyweight, organ weight, renal beta-adrenoceptor and myocardial beta- and myocardial alpha 1-adrenoceptor characteristics were investigated in female Sprague-Dawley rats after chronic subcutaneous (s.c.) administration of ethynyloestradiol (EE2, 0.2 microgram/day), levonorgestrel (NG, 2.0 micrograms/day) separately and in combination (EE2/NG). 2. EE2 caused a sustained increase in SBP from 6 weeks (maximum at 14 weeks, +22 mmHg compared to control) which was accompanied by increased kidney and ventricle weight after 12 weeks. EE2/NG-treated rats also demonstrated a gradual rise in SBP (maximum at 9 weeks, +18 mmHg compared with control) with renal and ventricular hypertrophy, but were normotensive by week 17 of treatment. In contrast, NG induced only transient SBP increases (maxima at 5 and 10 weeks, +14 mmHg compared with control), unaccompanied by organ hypertrophy. Norethisterone (2 micrograms/day) also produced transient increases (weeks 6-8, +13 mmHg) in SBP. 3. alpha 1- and beta-adrenoceptors were investigated using [3H]-prazosin and (-)-[125I]-iodocyanopindolol (ICYP), respectively. Myocardial alpha 1- and beta-adrenoceptors were unaffected by steroid contraceptive administration for up to 12 weeks. Renal beta-adrenoceptor affinity was markedly reduced in 12 week EE2-treated rats (equilibrium dissociation constant, KD, 53 +/- 7 pmol/L) compared with controls (KD, 31 +/- 4 pmol/L), an effect which was prevented by co-administration of NG (KD, 34 +/- 8 pmol/L). Renal beta-adrenoceptor number was not altered by any treatment. 4. The relatively late onset of organ hypertrophy and beta-adrenoceptor changes appear to result from, rather than cause, EE2-induced hypertension.

Effect of levonorgestrel and ethinyloestradiol on vasoconstriction in rat isolated vasculature

Female Sprague-Dawley rats were injected s.c. with 0.2 micrograms day-1 ethinyloestradiol (EE2) or 2.0 micrograms day-1 levonorgestrel (NG), procedures previously shown to increase systolic blood pressure. Increases in perfusion pressure to clonidine, phenylephrine, noradrenaline (NA) and angiotensin II (AII) were observed in rat isolated tail arteries, and to NA in the isolated mesenteric vasculature from steroid and vehicle-treated rats. NG treatment for four weeks produced increases in sensitivity to phenylephrine and NA in rat tail arteries; at 6 weeks the increases in sensitivity had largely disappeared but the maximum responses to clonidine and phenylephrine were increased. No change in sensitivity to AII was observed with NG. In contrast, EE2 treatment for six weeks produced increases in sensitivity to AII, and a decrease in sensitivity and maximum response to clonidine but not to phenylephrine or NA, in tail arteries. Responses to NA in the mesenteric vasculature were increased after 6 weeks NG treatment but unaffected after 12 weeks EE2 treatment. It is concluded that NG treatment stimulates alpha-adrenoceptor number, affinity or receptor-linked Ca2+ events which may contribute to its previously demonstrated hypertensive effect. The increased responsiveness to AII but not the decrease in alpha 2-adrenoceptor responsiveness may be associated with the chronic hypertension induced by EE2.