Increased tail artery vascular responsiveness to angiotensin II in cold-treated rats.

Chronic exposure of rats to cold for 1-3 weeks results in a mild form of hypertension. The renin-angiotensin system (RAS) has been implicated in this model of cold-induced hypertension. Previously we have characterized the vascular responsiveness in cold-acclimated animals, using aortic tissue, and recent studies have focused on the thermoregulatory responses of angiotensin II (AngII), utilizing the tail artery of the rat. Therefore in the current study we evaluated the vascular responsiveness of cold-treated rats to AngII in both aorta and tail artery at 2 and 4 weeks of cold exposure (5+/-2 degrees C). Systolic blood pressures were significantly elevated in cold-treated animals compared with control animals at both 2 and 4 weeks of cold exposure. At both of these time points body weights were reduced and ventricular weights were increased in cold-treated animals. After 2 weeks of cold exposure the vascular responsiveness of the aorta to AngII was significantly lower than that of controls. This vascular responsiveness to AngII was elevated and returned to control levels after 5 weeks of cold exposure. However, this pattern was not observed in the tail artery. The vascular responsiveness of tail artery rings from cold-treated rats to AngII was significantly greater than that of control animals during both 2 and 5 weeks of exposure to cold. The vascular contractile responses of both the aorta and tail artery to KCI in the cold-treated animals was not different from that of the control animals maintained at ambient room temperature, suggesting that the vascular smooth muscle contractile components were not altered by the cold exposure. Thus, the in vitro vascular reactivity to the receptor-mediated vasoconstrictor AngII was decreased in the sparsely innervated aorta and increased in the more densely innervated tail artery of the cold-treated animals when compared with controls. These results suggest that the increased responsiveness of AngII on the smooth muscle of the tail artery may play a role in adaptation to the cold and the maintenance of cold-induced hypertension.

Body fluid distribution in rats with cold-induced hypertension.

The purpose of this experiment was to determine body fluid distribution during chronic cold exposure and to further understand the mechanism of cold-induced hypertension. Blood pressures, hematocrit, and the plasma, blood, and extracellular fluid volumes were measured in rats at intervals of 1, 3, and 5 weeks after exposure to cold (5 degrees C). Resting systolic, diastolic, and mean blood pressures measured by direct arterial cannula were significantly elevated in a time-dependent manner over the duration of cold exposure. The increase in diastolic blood pressure, which reflects the peripheral vascular resistance, exceeded that of systolic blood pressure after both 3 and 5 weeks of exposure to cold. Pulse pressure was significantly decreased by 3 and 5 weeks of cold exposure. The plasma, blood, and extracellular fluid volumes were significantly increased after both 1 and 3 weeks of exposure to cold, but had returned to control levels by 5 weeks of cold exposure. Cold exposure, however, did not affect the hematocrit. The 2-h water intake after the cold-exposed rats were returned to warm (25 degrees C) (thermogenic drinking) was significantly increased compared to that of warm-acclimated rats during the first, third, and fifth week of exposure to cold. The present results suggest that the development of cold-induced hypertension is associated with blood volume expansion, and that the elevated blood pressure is maintained by increased peripheral vascular resistance without blood volume expansion. The results also imply that exposure to cold induces a dehydration in rats.

Reduction of cold-induced hypertension by antisense oligodeoxynucleotides to angiotensinogen mRNA and AT1-receptor mRNA in brain and blood.

Rats exposed chronically to mild cold (5 degrees C/41 degrees F) develop hypertension and cardiac hypertrophy. This provides a unique model of hypertension that is environmentally induced. The blood renin-angiotensin system (RAS) has been shown to play a role in both initiating and maintaining the high blood pressure (BP) in cold-induced hypertension. The mechanism also appears to involve both the tissue and brain RAS because there is increased mRNA for angiotensinogen (AGT) and angiotensin type 1 (AT1) receptors in brain and peripheral tissues, an increased spontaneous drinking response, and an increased dipsogenic response to acute administration of angiotensin II (Ang II) in cold-treated rats. Antisense oligodeoxynucleotides (AS-ODN), targeted to the RAS, have been shown to reduce BP in spontaneously hypertensive rats. Therefore, we injected AS-ODN in rats with cold-induced hypertension to test whether antisense inhibition was effective in reducing this nongenetic nonsurgical hypertension. Sprague-Dawley rats were made hypertensive by cold exposure and injected intracerebroventricularly with AS-ODN to AGT mRNA (n=6) or AT1 receptor mRNA (n=6). Systolic BP was recorded by tail cuff 24 hours later for 2 or 7 days, respectively. Systolic BP decreased significantly in response to AGT-AS-ODN (40+/-6 mm Hg, P<0.01) within 1 day after injection and to AT1 receptor-AS-ODN (P<0.05) for 3 days after injection. The maximum decrease was 41+/-10 mm Hg. Systolic BP then gradually increased to the preinjection level. The spontaneous drinking response to cold treatment also decreased significantly (P<0.05) after AGT-AS-ODN or AT1 receptor-AS-ODN intracerebroventricular injection. Intracardiac injection of AT1-AS-ODN (n=6) reduced systolic BP by 36+/-8 mm Hg (P<0.05) and decreased AT1 receptor as measured by autoradiography in aorta, adrenal glands, and kidneys 24 hours after injection. These data show that AS-ODN reduces BP in cold-induced hypertension and that the hypertension involves both peripheral tissues and central RAS in addition to blood-borne RAS mechanisms.

Aging and fluid homeostasis in rats.

The capacity of aging rats to defend body fluid homeostasis in response to a variety of dipsogenic and natriorexigenic stimuli was assessed. Male and female rats of both the Fischer 344 (FR) and Sprague-Dawley (SD) strains were used and tested at target ages of approximately 5, 10, 15, and 20 mo in both longitudinal and cross-sectional studies. There were no consistent age-related declines in water intake in response to water deprivation or acute administration of hypertonic NaCl; angiotensin (ANG) I, II, III; or isoproterenol. Likewise, there were no major impairments in either urinary excretion of the hypertonic NaCl load or excretion of water or hypotonic NaCl loads, although the latter were excreted more slowly in the older cohorts. The preference/aversion functions for NaCl solutions differed between SD and FR rats, but did not change with age except in male FR rats that lost their aversion to dilute NaCl at 20 mo of age. Intake of hypotonic NaCl solution after acute sodium depletion (furosemide treatment) showed a partial decline with age, and the older rats sustained larger estimated sodium deficits after a 6-h repletion period. A more complete age-related decline was observed in the intake of hypertonic NaCl stimulated by chronic dietary administration of a kininase II inhibitor (ramipril). Male rats of 15-20 mo of age showed no ramipril-induced sodium appetite. Brain ANG II receptor density, determined by autoradiography, declined by almost 50% in the paraventricular nucleus at 20 mo of age and declined slightly in the organum vasculosum laminae terminalis but did not decline in either the supraoptic nucleus or subfornical organ. Thus the major deficits in fluid intake in aging rats are related to salt appetite; the mechanism was not identified definitively.

Role of angiotensin in the dipsogenic effect of bradykinin in rats.

We have previously shown that peripheral administration of bradykinin (BK) induces water intake in rats acutely pretreated with captopril, a kininase II inhibitor. We now show that BK-induced drinking is also observed in rats treated chronically with dietary captopril, and that this is reversed by Hoe 140, a BK receptor antagonist. Both acute and chronic captopril in combination with BK caused a large increase in plasma renin activity. Fos-like immunoreactivity (Fos-in used as a marker of cellular activation) was induced by BK + captopril in regions of the brain previously associated with action of angiotensin (Ang) II, including the circumventricular organs and the magnocellular hypothalamic nuclei. However, while water intake induced by peripheral administration of Ang I was potentiated by acute administration of captopril, it was suppressed by chronic captopril treatment. Fos-IR induced in brain by Ang I was not markedly affected by either acute or chronic treatment with captopril. The simultaneous occurrence of potentiated drinking to BK and inhibited drinking to Ang I following chronic treatment with captopril suggest that different mechanisms of action are involved. In order to further examine this possibility, rats were given lesions of the anterodorsal third ventricle region. Lesions that completely abolished the water intake following administration of Ang II only partly attenuated water intake induced by BK + captopril. Further, Fos-IR induced by BK + captopril was only partly (31%) reduced in the supraoptic and paraventricular nuclei of lesioned rats compared with sham operated controls. We suggest that at least two mechanisms, one Ang-related, underlie drinking after BK+captopril.

Effect of chronic treatment with propranolol on the cardiovascular responses to chronic cold exposure.

The purpose of this study was to determine whether beta-adrenergic receptors are involved in the elevation of blood pressure, tachycardia, and cardiac hypertrophy in rats chronically exposed to cold (5 degrees C). Four groups of rats were used. Two groups of rats were exposed to 5 degrees C and the other 2 groups were kept at 25 degrees C. In each temperature condition, one group received regular food while the other received food to which a nonspecific beta-adrenoceptor antagonist, d,l-propranolol, was added. The blood pressure of the untreated, cold-exposed group increased significantly within 3 weeks of exposure to cold and remained elevated until the end of the experiment. Chronic treatment with d,l-propranolol decreased the rate of cold-induced elevation of blood pressure at doses of 1.0 and 1.2 g/kg of food, and produced a complete reversal of hypertension at a higher dose ( 1.5 g/kg of food). Propranolol also prevented tachycardia in cold-exposed rats. The fact that propranolol decreased the elevation of blood pressure and prevented tachycardia suggests that both beta1- and beta2-adrenoceptors are blocked in cold-exposed rats. Propranolol, however, did not affect cold-induced cardiac hypertrophy.

Cold-induced hypertension. A model of mineralocorticoid-induced hypertension.

Hypertension, tachycardia and cardiac hypertrophy develop in rats exposed to mild cold (5 degrees C, 41 degrees F) for 1 to 3 weeks. Elevation of blood pressure (BP) during cold exposure is sodium dependent, although the rats still have an elevation of BP with a minimum of NaCl in their diet. Drugs that interfere with the renin-angiotensin-aldosterone (RAA) system at various levels (propranolol, clonidine, captopril, losartan and spironolactone) are able to prevent the development of cold-induced hypertension (CIH). Plasma renin activity (PRA) increases during the first 3 weeks of exposure to cold and then gradually decreases toward control level. Increased blood pressure and dipsogenic sensitivity to administration of angiotensin II (Ang II) have been demonstrated during the first 3 weeks of exposure to cold suggesting an upregulation of Ang II receptors when PRA is elevated. Additional studies have shown greater Fos-like immunoreactivity in the diencephalon of cold-exposed compared to warm-acclimated rats after 1 hr i.v. infusion of Ang II (333 ng/kg/min). Thus, most characteristics of cold-induced hypertension mimic those of hypertension induced experimentally by chronic administration of large doses of deoxycorticosterone acetate (DOCA) and salt. The results suggest that CIH is a mineralocorticoid-induced hypertension and that various levels of the RAA system contribute.

Losartan inhibition of angiotensin-related drinking and Fos immunoreactivity in hypertensive and hypotensive contexts.

We assessed the ability of acute peripheral administration of the AT-1 receptor antagonist, losartan, to reverse both the water intake and Fos-immunoreactivity (Fos-ir) induced in rats by either peripheral or cerebroventricular (i.c.v.) administration of angiotensin (Ang) II. We compared this with endogenous generation of Ang II during either hypovolemia or hypotension. Relatively low doses of losartan blocked the dipsogenic effect of peripherally administered exogenous Ang II, but a higher dose (20 mg/kg) was needed to block the dipsogenic effect of i.c.v.-administered Ang II. Fos-ir induced by i.c.v. Ang II was attenuated in SFO and SON by 10-20 mg losartan/kg given peripherally, but Fos-ir in the MnPO and PVN was unaffected. These findings suggest that losartan has limited permeability into the brain. We used peripheral losartan to assess the contribution of Ang II to water intake and Fos-ir responses to peripheral injection of either polyethylene glycol (PEG; a colloid that produces non-hypotensive hypovolemia) or isoproterenol (hypotensive agent). Water intakes were unaffected by the higher dose of losartan given s.c. Intraperitoneal injection of EXP 3174, the active metabolite of losartan that may more readily penetrate the blood-brain barrier, inhibited isoproterenol-, but not PEG-induced water intakes. Fos-ir was induced by PEG and isoproterenol in several regions of the brain also activated by Ang II. Fos-ir was greatly attenuated in the SFO by losartan following administration of PEG, but not isoproterenol, and was either unaffected or increased in SON and PVN after either agent. These data suggest that the increased circulating Ang II following PEG or isoproterenol acts at the SFO and is more readily reversible by losartan in normotensive (PEG) than in hypotensive (isoproterenol) states. Non-Ang neural input to the SON and PVN, presumably from baroreceptors, appears to be sufficient to produce strong Fos-ir in these regions, as well as to engage drinking.

Correlative evidence of hypertension and altered cochlear microhomeostasis: electrophysiological changes in the spontaneously hypertensive rat.

The spontaneously hypertensive rat model has been used to show that hypertension is an important pathophysiological risk factor in age-related hearing loss. In the present study, compound action potential (CAP), electrochemical potential (ECP), and potassium concentration (CK+) measurements were taken from the cochlea of genetically predisposed, spontaneously hypertensive rats (SHR) and from normotensive Wistar-Kyoto (WKY) rats. In the SHR model, as the duration of hypertension increased with the animal's age (from 3 to 8 months), CAP thresholds increased, ECP increased in marginal cells only, and CK+ increased in both endolymph and marginal cells. Collectively, the data suggest that ionic alternations of cellular potentials are involved in hearing changes in the hypertensive state. Ultimately, such data may assist in understanding hearing loss in individuals who are diagnosed with hypertension.

Comparison of changes in blood pressure and dipsogenic responsiveness to angiotensin II in male and female rats chronically exposed to cold.

In most forms of experimentally induced hypertension in rats, females develop a less severe form of the disease than males. The objective of the present study was to compare the two genders with respect to the development of cold-induced hypertension. The results of the study indicate that both males and females develop comparable elevations of blood pressure and at approximately the same rate. Thus, the blood pressures of both groups increased significantly within 2 weeks of exposure to cold and reached similar maximal levels by the seventh week. The dipsogenic responsiveness of both groups of cold-exposed rats to acute administration of the peptide hormone, angiotensin II (AngII), was increased to approximately the same extent above that of warm-adapted counterparts, suggesting an increase in the responsiveness to AngII in the brain. To assess this possibility, the induction of the oncogene, cFos, was studied in brain following IV infusion of AngII (333 ng/kg/min). Fos-like immunoreactivity (FLI) was greater (p < 0.01) in subfornical organ, supraoptic and paraventricular hypothalamic nuclei of both cold-exposed groups compared to warm-adapted controls. Thus, both male and female rats have similar elevations of blood pressure as well as increased dipsogenic and FLI responsiveness to administration of AngII during chronic exposure to cold.

Angiotensin-related induction of immediate early genes in rat brain.

Several studies are reviewed in which behavioral aspects of angiotensin (Ang) II on fluid intake have been compared with induction of the immediate early gene product, Fos, as a marker of neuronal activation in rat bain. Either peripheral or central administration of Ang II induced Fos along the lamina terminalis (SFO, MnPO, AV3V) and in the magnocellular neurosecretory groups (SO, PVH). A similar pattern is seen with central injection of renin. Both pharmacological and antisense oligonucleotide probe studies indicate that an AT1 receptor is involved, probably with the initial transduction in the SFO. Treatments that induce sodium appetite all induce Fos along the lamina terminalis, but usually not in the SO or PVN. Kininase II inhibitors, such as captopril, acutely potentiate drinking to Ang I, but after chronic exposure they may inhibit water intake. In contrast, the dipsogenic effect of bradykinin which is manifest in the presence of acute captopril remains unaffected by chronic administration. This suggests that the sodium appetite that appears with chronic captopril treatment may depend in part on peptides other than Ang.

Age-related decline in thirst and sodium appetite in rats related to kininase II inhibition.

Water intake was examined in young (3-5 months) and old (20-24 months) rats following peripheral administration of either angiotensin (Ang) II or captopril+bradykinin (BK). Relative to body weight, intake after Ang II showed no age-related difference, while that after captopril+BK was markedly reduced in the old rats. In other rats that were not allowed to drink after captopril+BK, the induction of Fos-like immunoreactivity (IR) was much lower in the subfornical organ, supraoptic and median preoptic nuclei of old rats compared with their young counterparts. Intake of 0.15 M NaCl during chronic dietary administration of enalapril was robust in young rats but was much reduced in old rats, as was their plasma renin activity. Thus, reduced thirst and sodium appetite are found in conjunction with kininase II (angiotensin-converting enzyme) inhibitors in aging rats, possibly because they fail to generate the high levels of circulating renin seen in young rats under these conditions.

Centrally mediated vasodilation of the rat's tail by angiotensin II.

Acute peripheral administration of angiotensin II (AngII) to rats vasodilates the tail and reduces both metabolic rate and body temperature. To assess the role of the brain in these responses to AngII, a discrete lesion was aimed at the subfornical organ (SFO). Such lesions are known to abolish drinking and other responses to circulating AngII. A control group was sham-operated. Following recovery from surgery, dipsogenic responses to administration of AngII (150 micrograms/kg, SC) were tested. All lesioned rats failed to drink. One week later, the changes in colonic (TC) and tail skin (TSK) temperatures were measured following an identical injection of AngII. As reported before, control rats showed a substantial rise in in TSK (maximum rise 2.5 degrees C after 12 min). In contrast, the lesioned rats showed very little rise (0.3 degrees C) in TSK, significantly less than controls. The maximal drop in TC of the control group was slightly more (0.3 degrees C) than that of the lesioned group (0.2 degrees C), but the difference was not significant. The rats were, next, acutely exposed to cold (5 degrees C). Control rats showed a significantly larger decrease in TSK than lesioned rats. Lesions were verified both anatomically and using the functional metric of AngII-induced Fos-like immunoreactivity (Fos-IR). AngII induced strong Fos-IR in the SFO, median preoptic nucleus (MnPO), and in the magnocellular hypothalamic regions of control rats. The lesions ablated the anterior part of the SFO and dorsal MnPO, and greatly attenuated AngII-induced Fos-IR in the magnocellular hypothalamic regions. Thus, there appears to be central (SFO/MnPO) mediation of both the increase in TSK following administration of AngII and the decrease in TSK in the cold.

Expression of Fos in rat brain in relation to sodium appetite: furosemide and cerebroventricular renin.

Two experiments were performed to investigate the relationship between the expression of sodium appetite and the appearance of Fos-like immunoreactivity (Fos-IR) in the brain of rats. In the first experiment, rats were depleted of sodium by treatment with furosemide 24 h prior to sacrifice and without access to either food or sodium solution. Some rats had access to distilled water, and others had no fluids available during the 24 h. All of the furosemide-treated rats showed Fos-IR in both the subfornical organ (SFO) and around the organum vasculosum laminae terminalis (OVLT). Rats with access to distilled water during the depletion period showed no Fos-IR in the supraoptic (SON) or paraventricular hypothalamic nuclei (PVN) and, in parallel behavioral studies, comparably-treated rats consumed only 0.3 M NaCl solution at the end of the 24 h. In rats that had no fluids during the deprivation period, only about one half showed Fos-IR in SON and PVN and, in parallel behavioral studies, comparably treated rats consumed both water and 0.3 M NaCl solution at the end of 24 h. In a second experiment, cerebroventricular administration of renin stimulated short latency intake of 0.3 M NaCl and water. The relative intakes of water and NaCl were comparable at a low dose of renin, but intake of water exceeded that of NaCl after higher doses. Renin induced Fos-IR in SFO, MnPO, peri-OVLT region, SON and PVN. Both Fos-IR and fluid intake were antagonized by administration of losartan, an angiotensin II type 1 receptor antagonist. Thus, only the circumventricular organs of the lamina terminalis showed Fos-IR during each natriorexigenic regimen in these studies. These data support the view that Ang II of both central and peripheral origin activates the SFO and/or peri-OVLT region and contributes to sodium appetite.

Relationship between drinking and increase in tail skin temperature of rats treated with L-5-hydroxytryptophan.

Administration of L-5-hydroxytryptophan (25 micrograms/kg, s.c.), the immediate precursor of serotonin, to rats induces drinking and increases tail skin temperature. Decreases in both the metabolic rate (rate of oxygen consumption) and colonic temperature also occur. These responses are similar to those observed following acute administration of angiotensin II. However, both drinking and the increase in tail skin temperature are unaffected by prior administration of the non-peptide angiotensin II, AT-1 receptor antagonist, losartan potassium, but are partially attenuated by the beta-adrenoceptor antagonist, propranolol. This suggests that the responses are independent of the angiotensin AT-1 receptor but may be mediated, in part at least, by the beta-adrenoceptor. The hypothesis is presented that when either L-5-hydroxytryptophan or angiotensin II is administered to rats, they act centrally to reduce the set-point for body temperature regulation, thus resulting in the reflex activation of mechanisms that reduce body temperature, including an increase in tail skin temperature and a decrease in metabolic rate.

Effect of administration of angiotensin II and isoproterenol, alone and in combination, on drinking and tail skin temperature of the rat.

Both angiotensin II (AII) and the beta-adrenergic agonist isoproterenol are well known to induce drinking and increase tail skin temperature (TSK) in rats. Previous studies have shown that these two compounds have an additive, rather than interactive, dipsogenic effect when administered together. The present studies confirmed the additivity of the dipsogenic effect of the two compounds and attempted to extend these results to TSK. The results reveal that at a series of dosage combinations of AII [100 and 125 micrograms/kg, subcutaneously (SC)] and isoproterenol (ISOP) (10, 12.5, and 25 micrograms/kg, SC) administered together, the increase in TSK usually induced by each compound was canceled. This occurred at the lower doses of ISOP in combination with AII. Thus, the differences between dipsogenic and thermoregulatory effects of combined doses of AII and ISOP suggest a difference in central integration. Important differences between the thermoregulatory responses to administration of the two compounds include increases in metabolic rate and colonic temperature when ISOP is administered and decreases in both when AII is administered. It is suggested that the effect of the two compounds to cancel each other with respect to TSK is related to their opposite effects on metabolic rate.

Increased dipsogenic responsiveness to angiotensin II in rats exposed to cold: rate of loss after return to thermoneutral ambient temperature.

Subcutaneous administration of angiotensin II (Ang II) to rats exposed chronically to 5 degrees C induced an increased drinking response compared with that of warm-acclimated controls. The exaggerated drinking response was also observed when graded doses of Ang II were administered into the lateral cerebroventricle (icv) of chronically cold-exposed rats. A maximal drinking response occurred in cold-treated, but not in control, rats when the lowest dose of Ang II (1.6 ng/rat) was administered icv. Thus, it is clear that the dipsogenic responsiveness to either centrally or peripherally administered Ang II is increased by chronic exposure to cold. To assess whether the increased responsiveness was retained after removal from cold, graded doses of Ang II were administered to rats removed from cold to a thermoneutral environment. The results again showed a maximal responsiveness to the lowest dose of Ang II administered (25 micrograms/kg, sc) to cold-treated rats that had either just been removed from cold or removed from cold 2 hr prior to treatment. Cold-exposed rats had an ED50 for Ang II-induced drinking that was about half that of their warm-acclimated controls. To assess how long the cold-induced increase in dipsogenic responsiveness to Ang II lasted after return to a thermoneutral temperature, rats were removed from cold for 24, 48, or 60 hr and then administered graded doses of Ang II (25, 50, or 100 micrograms/kg, sc). The results suggest that between 46 and 52 hr after removal from cold, the cold-induced increase in dipsogenic responsiveness to Ang II returned to the level of the controls. Hence, the physiological changes in the dipsogenic mechanism induced by exposure to cold are not immediately reversible when the rats are returned to a thermoneutral ambient temperature.

Effect of pyridoxine and tryptophan, alone and combined, on the development of deoxycorticosterone acetate-induced hypertension in rats.

Chronic dietary administration of pyridoxine HCl (300 mg/kg/day), L-tryptophan (1.26 g/kg/day), or a combination of the two can attenuate the elevation of systolic blood pressure in DOCA-salt-treated rats. With these treatments, the characteristic increase in the weight of the heart accompanying chronic administration of DOCA (786 micrograms/kg/day) was also attenuated. Thus, both tryptophan and pyridoxine possess antihypertensive properties, and the combination of the two appeared to provide greater protection than either alone. The results are consistent with the possibility that pyridoxine, an important cofactor in the metabolic pathways for tryptophan, may facilitate the conversion of tryptophan to antihypertensive compounds. Additional studies will be required to determine which of the metabolites of tryptophan possess antihypertensive properties. Pyridoxal phosphate, one of the metabolites of pyridoxine, was also administered chronically in the diet (1.0 and 2.0% by weight) to rats whose blood pressures were elevated by administration of DOCA. The results of this study suggest that pyridoxal phosphate can also lower the blood pressure of rats with established hypertension. Thus, these studies reveal that pyridoxine, pyridoxal phosphate and tryptophan are potential antihypertensive agents.