Effects of phenylalanine and phenylpyruvate on ATP-ADP hydrolysis by rat blood serum.

The nucleotide (ATP-ADP)/nucleoside (adenosine) ratio in the circulation can modulate the processes of vasoconstriction, vasodilatation and platelet aggregation. The main objective of the present study with rat blood serum was to evaluate the possibility of changes in nucleotide hydrolysis by phenylalanine (Phe) and phenylpyruvate (PP), the levels of which could increase in the circulation of individuals with phenylketonuria. Results demonstrated that Phe in the range 1.0-5.0 mM inhibited the ADP hydrolysis by rat serum. The effect of inhibition by Phe on ATP hydrolysis appeared only at a concentration of 5.0 mM. PP had no significant effect upon nucleotide hydrolysis. Kinetic analysis indicated that the inhibition of ADP and ATP hydrolysis by Phe in rat blood serum is uncompetitive. Conversely, Phe and PP did not affect the hydrolysis of p-nitrophenyl-5'-TMP by rat serum.

Central inhibition of AT1receptors by eprosartan--in vitro autoradiography in the brain.

All components of the renin-angiotensin system have been demonstrated in the brain and AT1 receptors have been localized in brain areas involved in central cardiovascular regulation. It is currently unclear whether AT1 receptor antagonists, which are increasingly used in the treatment of arterial hypertension and chronic heart failure, have the potential to mediate action via the central renin-angiotensin system. Therefore, we tested the in vivo access of the non-peptide AT1 receptor antagonist, eprosartan (30 and 60 mg per kg of body weight (BW) for 4 weeks, i.p. administered by osmotic minipumps), to angiotensin II receptors in the rat brain by in vitro autoradiography with 125I- (Sar1- Ile8) angiotensin II as a ligand. Eprosartan significantly increased plasma renin activity by four-fold and six-fold at doses of 30 and 60 mg x kg(-1), respectively (P< 0.05 vs CTRL). In the brain, eprosartan produced a dose-dependent inhibition of AT receptor binding in the median cerebral artery ( 850 +/- 249 and 650 +/- 106 vs 1072 +/- 116 dpm x mm(-2) of CTRL; P< 0.05). Furthermore, eprosartan inhibited angiotensin II receptor binding in discrete brain areas, which express exclusively, or predominantly, AT1 receptors both outside and within the blood-brain barrier, such as the paraventricular nucleus ( 180 +/- 47 and 130 +/- 18 vs 545 +/- 99 dpm x mm(-2)of CTRL; P< 0.05), the subfornical organ ( 106 +/- 26 and 112 +/- 17 vs 619 +/- 256 dpm x mm(-2)of CTRL; P< 0.05), and the organum vasculosum laminae terminalis ( 461 +/- 110 and 763 +/- 136 vs 1033 +/- 123 dpmx mm(-2)of CTRL; P< 0.05). These results emphasize that eprosartan readily crosses the blood-brain barrier in vivo and selectively inhibits binding to AT1 receptors in specific brain nuclei. The modulation of central regulatory mechanisms might contribute to AT1 receptor antagonists overall therapeutic efficacy in cardiovascular disease.

Chronic ACE inhibition by quinapril modulates central vasopressinergic system.

OBJECTIVE: The role of the brain as a target for angiotensin converting enzyme (ACE) inhibitors in the treatment of heart failure and hypertension is unclear. To test the hypothesis that ACE inhibitors may modulate other central neuropeptide systems such as the central vasopressin system, we studied the effects of chronic treatment with the ACE inhibitor, quinapril, on ACE activity and on central vasopressin content in specific brain areas in rats. METHODS: 22 rats were chronically treated with quinapril (6 mg.kg-1 BW per gavage daily for 6 weeks; untreated controls, n = 14). ACE density in various brain regions was assessed by in vitro autoradiography using the specific ACE inhibitor, 125I-351A. Vasopressin content was determined in 19 brain areas (micropunch technique) known to be involved in cardiovascular regulation. RESULTS: Following chronic quinapril treatment ACE was significantly decreased in the thalamus (-38%), hypothalamus (-37%), hypophysis (-35%), cerebellum (-36%) choroid plexus (-20%), and locus coeruleus (-35%). Additionally, a marked reduction in serum ACE activity (-97%) was observed. Plasma levels of vasopressin were significantly decreased after quinapril treatment (0.97[s.e.m. 0.11] vs. 1.63[0.24] pg.ml-1 in controls, P < 0.05). Vasopressin content was significantly reduced in 9 of 19 specific brain areas. Regarding the hypothalamic vasopressin-producing nuclei, vasopressin was decreased in the paraventricular (292[197] vs. 2379[585] pg.mg-1 crotein in controls; P < 0.001) and supraoptic nuclei (13618[1979] vs. 24525[3894] pg.mg-1 protein; P < 0.05), but not in the suprachiasmatic nucleus. Vasopressin content was significantly reduced in brain areas connected by vasopressinergic fibres originating in the hypothalamic paraventricular nucleus: namely central gray, subcommissural organ, organum vasculosum laminae terminalis, dorsal raphe nucleus, and locus coerules. Vasopressin content was also significantly reduced in the median eminence (5887[1834] vs. 28321[4969] pg.mg-1 protein, P < 0.001), where the hormone is mainly concentrated in the hypothalamo-hypophysial tract. CONCLUSIONS: Autoradiographic studies in vitro indicate that orally administered quinapril suppresses central ACE activity after chronic treatment. ACE inhibition by quinapril strongly influences vasopressin content in important brain areas which are involved in central cardiovascular regulation. Therefore, central modulatory effects of ACE inhibitors may also contribute to overall therapeutic efficacy.