Cobalt chloride toxicity elicited hypertension and cardiac complication via induction of oxidative stress and upregulation of COX-2/Bax signaling pathway

Cobalt is a ferromagnetic metal with extensive industrial and biological applications. To assess the toxic effects of, and mechanisms involved in cobalt chloride (CoCl₂)-induced cardio-renal dysfunctions. Male Wistar rats were exposed orally, daily through drinking water to 0 ppm (control), 150 ppm, 300 ppm, and 600 ppm of CoCl₂, respectively. Following exposure, results revealed significant ( p < 0.05) rise in markers of oxidative stress, but decreased activities of catalase, glutathione peroxidase, glutathione-S-transferase, and reduced glutathione content in cardiac and renal tissues. There were significant increases in systolic, diastolic, and mean arterial blood pressure at the 300- and 600-ppm level of CoCl₂-exposed rats relative to the control. Prolongation of QT and QTc intervals was observed in CoCl₂ alone treated rats. Also, there were significant increases in the heart rates, and reduction in P wave, and PR duration of rats administered CoCl₂. Histopathology of the kidney revealed peritubular and periglomerular inflammation, focal glomerular necrosis following CoCl₂ exposure. Further, cyclooxygenase-2 and B-cell associated protein X expressions were upregulated in the cardiac and renal tissues of CoCl₂-exposed rats relative to the control. Combining all, results from this study implicated oxidative stress, inflammation, and apoptosis as pathologic mechanisms in CoCl₂-induced hypertension and cardiovascular complications of rats.

Effect of arsenic acid withdrawal on hepatotoxicity and disruption of erythrocyte antioxidant defense system

We investigated the effects of withdrawal from Sodium arsenite (NaAsO₂) on the hepatic and antioxidant defense system in male Wistar rats using a before and after toxicant design. Rats were orally gavaged daily with varying doses of NaAsO₂ for a period of 4 weeks. One half of the population was sacrificed and the remaining half had the toxicant withdrawn for another further 4 weeks. Biochemical and immunohistochemical techniques were used to assess the impact of withdrawal on the erythrocyte and hepatic systems. Exposure of Wistar rats to NaASO₂ led to a significant (p < 0.05) increase in hepatic and erythrocyte markers of oxidative stress (malondialdehyde, thiol contents and hydrogen peroxide generation). Concurrently, there was a significant (p < 0.05) increase in hepatic and erythrocyte antioxidant enzymes (glutathione-S-transferase, glutathione peroxidase and superoxide dismutase) following exposure. Withdrawal from NaAsO₂ exposure led to a decline in both erythrocyte and hepatic markers of oxidative stress and together with a significant improvement in antioxidant defense system. Histopathology and immunohistochemistry revealed varying degrees of recovery in hepatocyte ultrastructure alongside increased expression of the pro-survival protein Kinase B (Akt/PKB) after 4 weeks of NaAsO₂ withdrawal. Conclusively, withdrawal from exposure led to a partial recovery from oxidative stress-mediated hepatotoxicity and derangements in erythrocyte antioxidant system through Akt/PKB pathway.

Enhanced pial arteriolar sensitivity to bioactive agents following exposure to endothelin-1

Effects of prior exposure of pial arterioles to endothelin-1 (ET-1) (10(-9) M) on the constriction induced by the by-products of hemolyzed blood (5-HT, LTC4, LPA, and thromboxane analog U-46619) were examined. Piglets (age: 1-3 d) anesthetized with a mixture of ketamine hydrochloride and acepromazine were implanted with cranial windows, and anesthesia was maintained with alpha-chloralose. Topical applications of the by-products of hemolyzed blood mildly constricted pial arterioles. Following prior exposure of the microvessels to ET-1, application of the by-products of hemolyzed blood produced significantly potentiated and long-lasting constrictions compared to the controls. In another experiment, pretreatment of pial arterioles with U-46619 (10(-8) M) also potentiated the constriction induced by ET-1. The constriction produced was fast and longer-lasting. Thus, these data show that by-products of hemolyzed blood, though not potent vasoconstrictors per se, potently constricted pial arterioles in the presence of ET-1. The same agents in the CSF can also potentiate constriction induced by ET-1. Hence, by-products of hemolyzed blood may play a significant role in the initiation and maintenance of cerebral arterial narrowing observed following intracranial bleeding.

Regulation of ET-1 biosynthesis in cerebral microvascular endothelial cells by vasoactive agents and PKC

Endothelin-1 (ET-1) is the most potent vasoconstrictor agent known. ET-1 is elevated in the cerebrospinal fluid following hemorrhage and brain injury and can compromise cerebral microvascular homeostasis. The modulation of ET-1 production by cerebral microvascular endothelial cells and the mechanism by which such changes take place are very important in our understanding of the pathological roles of ET-1. In the present study, we investigated the effects of vasoconstrictor agents that can be released from hemolyzed blood, cAMP-dependent dilators, and the role of protein kinase C (PKC) in the regulation of ET-1 production by piglet cerebral microvascular endothelial cells in culture. ET-1 was measured by RIA. 1) Cerebral microvascular endothelial cells synthesize and release ET-1 into the media; 2) 5-hydroxytryptamine (5-HT), lysophosphatidic acid (LPA), thromboxane analog U-46619, fetal bovine serum (20%), and phorbol 12-myristate 13-acetate significantly increase ET-1 production; 3) basal and vasoconstrictor agent-induced increases in ET-1 production by endothelial cells may be mediated via PKC; 4) cAMP-dependent vasodilators attenuate the basal production of ET-1 by cerebral microvessels; and 5) pretreatment of endothelial cells with a higher concentration of LPA, U-46619, or 5-HT counterbalances the cAMP-dependent dilator agent-induced reduction in basal ET-1 production. Therefore, by-products of hemolyzed blood can stimulate the production of ET-1 by a PKC-mediated mechanism. cAMP-dependent dilators can attenuate the vasoconstrictor agent-induced elevation in ET-1 production. These results suggest that cerebral microvascular homeostasis could be compromised by effects of interactions among vasoactive agents released during conditions injurious to the brain and they may further the understanding of potential contributions of hemolyzed blood clots to subarachnoid hemorrhage-induced vasospasm.

Role of lysophosphatidic acid in endothelin-1- and hematoma-induced alteration of cerebral microcirculation

Cerebral hematoma increases cerebrospinal fluid (CSF) endothelin-1 (ET-1). Inhibitors of ET-1 synthesis prevent this increment and hematoma-induced modification of cerebral arteriolar reactivity. We hypothesized that intrathecal ET-1 injection could 1) modify pial arteriolar reactivity similarly to hematoma; 2) increase CSF lysophosphatidic acid (LPA), a potential contributor to altered cerebrovascular reactivity; and 3) reduce the level of adenosine 3',5'-cyclic monophosphate (cAMP) in the CSF. Either ET-1 (10(-7) M) or artificial CSF was injected over the left parietal cortex of newborn pigs. Four days later, cranial windows were implanted. CSF ET was increased from a basal level of 11 fmol/ml to 18 fmol/ml 4 days after ET-1 injection, whereas CSF cAMP was reduced from 2,700 to 950 fmol/ml. The mean diameter of pial arterioles was reduced 31%. In control animals, 10(-12) M ET caused dilation, and higher concentrations induced vasoconstriction. Four days after ET-1 injection topical ET-1 caused constriction instead of dilation at 10(-12) M, and constrictions at higher doses were enhanced. Norepinephrine-induced constrictions were potentiated in the ET-1-injected group. Dilations to cAMP-dependent (but not independent) vasodilators were attenuated after ET-1. The concentration of the vasoconstrictor lipid mediator LPA increased approximately fourfold. Thus intrathecal injection of ET-1 mimics hematoma-induced modification of cerebral vascular reactivity and increase in LPA production. The mechanism(s) of ET-1- and hematoma-induced modifications may involve LPA, which is known to contribute to the loss of dilator responses by inhibition of cAMP product on. The present study further suggests that ET-1 together with LPA could be causing changes in cerebrovascular reactivity following cerebral hemorrhage. ET-1 stimulates the release of LPA from brain parenchyma independent of serum so that LPA could serve as a secondary mediator.

5-Hydroxytryptamine-induced vasoconstriction after cerebral hematoma in piglets

Subarachnoid hematoma produces cerebral vasoconstriction that may lead to death or permanent disability. After hematoma, enhanced pial arteriolar responses to vasoconstrictor agents have been reported in newborn pigs. The present study was designed to address the hypothesis that 5-hydroxytryptamine (5-HT) constricts piglet pial arterioles, and hematoma augments this constriction. Piglets (1-3 d old) anesthetized with ketamine and acepromazine received either 3 mL of artificial cerebrospinal fluid (control) or autologous nonheparinized blood (hematoma) injected onto the cortical surface. Four days after injection, closed cranial windows were implanted over the injected area under alpha-chloralose anesthesia. Vascular reactivity to 5-HT was examined. In control piglets, topical application of 5-HT (10(-9), 10(-7), and 10(-5) M) induced very mild, dose-dependent constriction of pial arterioles (-6 +/- 1, -10 +/- 2, and -12 +/- 4%, respectively). These constrictions were substantially augmented in piglets with hematoma (-12 +/- 2, -19 +/- 1, and -30 +/- 2%, respectively). After topical application of 5-HT, cerebrospinal fluid samples were collected from under the window to determine the effects of 5-HT on the levels of 6-keto-prostaglandin F1 alpha and thromboxane B2. The baseline levels of 6-keto-prostaglandin F1 alpha and thromboxane B2 before 5-HT were 1791 +/- 387 and 434 +/- 74 pg/mL, respectively, in the control. 5-HT application had no significant effects on these prostanoid levels (levels at the highest concentration of 5-HT had a corresponding value of 1175 +/- 301 and 288 +/- 74 pg/mL for 6-keto-prostaglandin F1 alpha and thromboxane B2, respectively). However, indomethacin (5 mg/kg, i.v.) treatment of the control piglets potentiated the constriction in response to 5-HT (-11 +/- 1, -15 +/- 2, and -24 +/- 3%, respectively) sufficiently to produce constriction similar to that in the hematoma group. 5-HT has little effect on normal pial arterioles of newborn piglets but is a more potent cerebral vasoconstrictor in conjunction with cerebral hematoma.

Role of endothelin-1 in cerebral hematoma-induced modification of cerebral vascular reactivity in piglets

Endothelin-1 (ET-1) has been implicated in hematoma-induced cerebral vasoconstriction and modification of cerebral microvascular reactivity, particularly attenuation of vasodilation to cAMP-dependent dilators and enhanced vasoconstriction to ET-1. We examined effects of the ET-1 antagonist, BQ-123, on hematoma-induced modification of pial arteriolar responses to ET-1 and iloprost, a cAMP-dependent dilator, in vivo, plus the effects of such treatment on the cortical CSF cAMP. Closed cranial windows were implanted in alpha-chloralose anesthetized piglets 4 days following cortical subarachnoid injection of: (1) artificial cerebrospinal fluid (aCSF); (2) autologous blood; (3) BQ-123 alone; or (4) BQ-123 in combination with blood. ET-1 in CSF was significantly elevated from 3 in control to 45 fmol/ml 6 h following hematoma, dropping to 24 fmol/ml at 24 h but remaining above control 4 days later (14 fmol/ml). The mean diameters of pial arterioles were reduced 30% 4 days following blood injection. This reduction was prevented by pretreatment with BQ-123. In the control piglets, pial arterioles dose-dependently dilated to topical application of iloprost with increases in diameter of 10%, 16% and 21% at 10(-12) M, 10(-10) M and 10(-8) M, respectively. Iloprost-induced dilation was attenuated by hematoma to 4%, 9% and 14% at 10(-12) M, 10(-10) M and 10(-8) M, respectively. Treating piglets with BQ-123 along with hematoma on day 1 prevented the hematoma-induced attenuation of pial arteriolar dilation to iloprost on day 4 (14%, 21% and 29% at 10(-12) M, 10(-10) M and 10(-8) M, respectively). Conversely, dilation to sodium nitroprusside (SNP) was not different among the groups. Topical ET-1 dilated pial arterioles at 10(-12) M and produced dose-dependent constriction at higher doses in the control piglets. The dilation at 10(-12) M ET-1 was reversed to constriction 4 days following hematoma and constrictions to higher doses were enhanced. BQ-123 treatment along with hematoma prevented both the loss of low dose dilation and the enhanced vasoconstriction to ET-1. Treatment with BQ-123 alone on day 1 did not affect the dilation to iloprost or constriction to ET-1, 4 days later. The cortical CSF level of cAMP was significantly reduced from 1637 fmol/ml in controls to 294 fmol/ml in piglets with hematoma. Treatment with BQ-123 along with hematoma blocked the reduction in cAMP (3369 fmol/ml). Initial elevation of ET-1 and the subsequent activation of ETA, receptor may play an important role in hematoma-induced alterations of cerebral vascular reactivity and prolonged cerebral vasoconstriction that occur 4 days later. Thus, cerebral hematoma appears to attenuate iloprost-induced dilation and reduce basal cAMP level 4 days following hematoma via release that involves ET-1 of substance(s) on day 1 of hematoma. This substance(s) may act by inhibiting adenylyl cyclase. These results suggest that ET-1 plays an important role in the blood-induced prolonged cerebral vasoconstriction and altered vasoreactivity that follows cerebral hemorrhage via stimulation of ETA receptor.

Hematoma-induced enhanced cerebral vasoconstrictions to leukotriene C4 and endothelin-1 in piglets: role of prostanoids

Cerebral hematoma enhances vasoconstriction induced by topical application of the vasoconstrictor agents endothelin-1 (ET-1) and leukotriene C4 (LTC4). We investigated the influence of dilator prostanoids on vasoconstrictions induced by ET-1 and LTC4 in piglets. Newborn pigs anesthetized with alpha-chloralose were fitted with closed cranial windows 4 d after cortical subarachnoid injections of artificial cerebrospinal fluid (aCSF) (control) or blood (hematoma). The responsiveness of pial arterioles to topical application of the vasoconstrictors ET-1 and LTC4 was examined in the control and hematoma groups before and after treatment with indomethacin (5 mg/kg, i.v.). Vasoconstriction to topical application of LTC4 and ET-1 was enhanced by hematoma compared with the control (28 +/- 2% versus 21 +/- 2% for 10(-8) M LTC4 and by 25 +/- 2% versus 15 +/- 1% for 10(-8) M ET-1, respectively). The lower dose of ET-1 (10(-12) M) dilated pial arterioles in the control group by 6 +/- 2%, hematoma blocked this dilation and it was converted to constriction (10 +/- 1%). Indomethacin treatment enhanced vasoconstriction to LTC4 in the control group to a similar constriction to that observed in the hematoma group. Indomethacin treatment also enhanced vasoconstriction to ET-1 in the control group (25 +/- 1% for 10(-8) M) to similar constrictions to those observed in the hematoma group (25 +/- 2% for 10(-8) M). Dilation to the lower dose of ET-1 was blocked and converted to constriction (17 +/- 2% for 10(-12) M) by indomethacin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Subarachnoid hematoma attenuates vasodilation and potentiates vasoconstriction induced by vasoactive agents in newborn pigs

The effects of perivascular blood on pial arteriolar vasoreactivity to selected vasodilators and vasoconstrictors were examined in vivo in a newborn pig model. alpha-Chloralose-anesthetized newborn pigs were fitted with closed cranial windows 4 d after cortical subarachnoid injections of autologous blood. The responsiveness of pial arterioles to topical application of dilator agents [iloprost, prostaglandin E2 (PGE2), histamine, and sodium nitroprusside (SNP)] and vasoconstrictor agents [leukotriene C4 and endothelin-1 (ET-1) in artificial cerebrospinal fluid was studied in control and blood-injected piglets. Pial arterioles dilated dose dependently in response to topical application of iloprost, PGE2, histamine, and SNP in the control group, with increases in diameter of 54, 44, 67, and 50% at 10(-8) M, 10(-5) M, 10(-5) M, and 10(-5) M, respectively. These dilations in response to iloprost, PGE2, and histamine in the blood-injected piglets were significantly attenuated to 23, 18, and 34%, respectively, whereas the dilation in response to SNP was not changed (64%). Constrictions in response to 10(-8) M leukotriene C4 and ET-1 were 16 and 26% and were potentiated by hematoma to 36 and 43%, respectively. The lowest dose of ET-1 (10(-12) M) significantly dilated pial arterioles in the control but not in the blood-treated group. We conclude that prolonged exposure of pial arterioles to perivascular blood attenuates cerebrovascular dilation in response to selected vasoactive agents (iloprost, PGE2, and histamine) but not to SNP, suggesting that blood-induced attenuation of vasodilation and the generalized vasoconstriction may involve inhibiting the prostanoid/cAMP signaling pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in Trypanosoma cruzi infectivity by treatments that affect calcium ion levels

The possible role of the intracellular Ca2+ level in the regulation of Trypanosoma cruzi infectivity was explored by measuring the capacity of trypomastigote forms of this organism to invade mammalian host cells after treatments which decrease or elevate cytoplasmic Ca2+. Parasites loaded with either bis-(o-aminophenoxy)-ethane-N,N,N',N' tetraacetic acid (BAPTA) or 2-([2-bis(carboxymethyl)-amino-5-methylphenoxy]methyl)-6-nethoxy-8 - bis(carboxymethyl)aminoquinoline (Quin-2) to chelate Ca2+ displayed significantly decreased infectivity. This effect was denoted by reductions in both the proportion of rat heart myoblasts invaded by the parasite in vitro and the number of trypanosomes penetrating these host cells, the extents of which were BAPTA or Quin-2 concentration dependent. Consistent with these observations, inhibitory effects were also recorded when the parasite was pretreated with the calmodulin-binding phenothiazines trifluoperazine and chlorpromazine or with felodipine, a chemically different type of calmodulin antagonist, for as little as 5 min. In contrast, pretreatment with the Ca2+ ionophore ionomycin, which elevated Ca2+ levels in T. cruzi, significantly enhanced the infective capacity of the parasite. These results point to the existence of a Ca(2+)-dependent mechanism that regulates the invasive capacity of T. cruzi.

Inhibition of S-adenosyl-L-methionine (AdoMet) decarboxylase by the decarboxylated AdoMet analog 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811) decreases the capacities of Trypanosoma cruzi to infect and multiply within a mammalian host cell

A decarboxylated S-adenosyl-L-methionine (AdoMet) analog, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), that specifically and irreversibly inhibits AdoMet decarboxylase (DC) was used to investigate the role of AdoMetDC in the regulation of host cell invasion and intracellular replication by Trypanosoma cruzi. The presence of MDL 73811 in cocultures of T. cruzi and rat heart myoblasts (RHM) significantly inhibited host cell infection in a dose-dependent manner. This effect, evidenced by reductions in the proportion of infected RHM and the number of parasites per 100 RHM, was due to MDL 73811 action on T. cruzi as it was reproduced when the parasites, but not the RHM, were pretreated with the inhibitor. Significant inhibition of infectivity required a 10-min treatment with MDL 73811 although greater effects ensued with additional incubation time. We could not detect signs of recovered infectivity up to 6 hr after the removal of nonincorporated MDL 73811, suggesting that either AdoMetDC turnover in T. cruzi is a relatively slow process or the amounts of MDL 73811 taken up could not be blocked or eliminated during this time period. MDL 73811-mediated inhibition of infectivity was not bypassed by the addition of exogenous spermidine or spermine, suggesting that the mechanism of action does not involve reduced production of these polyamines due to AdoMetDC inhibition. Intracellular accumulation of AdoMet appeared to be a more plausible explanation in view of the fact that exogenous AdoMet significantly inhibited infectivity. When added to infected RHM cultures, MDL 73811 or AdoMet inhibited also intracellular T. cruzi growth.(ABSTRACT TRUNCATED AT 250 WORDS)

DL-alpha-difluoromethylarginine inhibits intracellular Trypanosoma cruzi multiplication by affecting cell division but not trypomastigote-amastigote transformation

DL-alpha-difluoromethylarginine (DFMA), a specific, irreversible inhibitor of arginine decarboxylase (ADC), decreases the capacity of Trypanosoma cruzi to invade and multiply within different types of mammalian host cells in vitro. In this work we found that inhibition of intracellular growth results from selective impairment of amastigote division without appreciable alteration of the capacity of the invading trypomastigotes to transform into the replicative amastigote form. Addition of agmatine, the product of arginine decarboxylation, reversed the inhibitory effect of DFMA. Inhibition of ornithine decarboxylase activity by DL-alpha-difluoromethylornithine present in the medium prior to and during infection did not affect trypomastigote transformation or amastigote replication and did not change the magnitude of the inhibitory effect of DFMA on parasite multiplication. Hence, neither polyamine synthesis via the ornithine decarboxylase pathway nor salvage of host cell polyamines by T. cruzi appeared to be a likely explanation for the normal rate of parasite transformation that was seen in the presence of DFMA. Two clones of T. cruzi, TMSU-1 and TMSU-2, were tested for their degrees of sensitivity to the inhibitory effects of DFMA. Both trypomastigote association with (i.e., binding to and penetration of) myoblasts, and intracellular amastigote multiplication by either clone were found to be significantly (P less than 0.05) but not completely inhibited by DFMA. Therefore, the partial inhibition of T. cruzi infectivity and replication caused by DFMA is unlikely to represent a composite of effects of the drug on DFMA-sensitive and insensitive clones.(ABSTRACT TRUNCATED AT 250 WORDS)

Do centrally-acting antihypertensive drugs act at non-adrenergic as well as alpha-2 adrenoceptor sites?

Rabbits were treated with guanabenz, clonidine and rilmenidine for 6 days via osmotic minipumps. Blood pressure, heart rate and responses to intracisternal clonidine were measured after 1 and 6 days treatment. Radioligand binding to forebrain and hindbrain membranes after 6 days treatment was examined using [3H]yohimbine to measure the number of adrenergic binding sites and [3H]clonidine and [3H]idazoxan to assess nonadrenergic imidazoline sites. No change in nonadrenergic imidazoline binding was observed but adrenergic binding was decreased in forebrain and hindbrain by guanabenz and in hindbrain by clonidine treatment. Resting heart rate was decreased after 1 day's treatment with partial recovery by day 6. At this time heart rate significantly reduced in the clonidine and rilmenidine treated groups but not the guanabenz group. No significant change in baseline blood pressure was observed in normotensive rabbits. Both depressor and bradycardia responses to intracisternal clonidine were attenuated after 1 day's dosing but only depressor responses were influenced after 6 days. Blood pressure and heart rate thus appeared to be regulated independently. It is possible that imidazoline receptors predominate in the central control of blood pressure while central alpha-2 adrenoceptors play a greater part in heart rate regulation.

Imidazole binding sites in rabbit kidney and forebrain membranes

1. The binding of [3H]-clonidine, [3H]-idazoxan and [3H]-yohimbine to rabbit forebrain and kidney membranes was compared. 2. Yohimbine bound exclusively to adrenergic sites, idazoxan to non-adrenergic sites and clonidine to both non-adrenergic and adrenergic sites. 3. Differences were observed between the ligands not only in binding at adrenergic and non-adrenergic sites but also between the non-adrenergic binding of [3H]-clonidine and [3H]-idazoxan. 4. However, no tissue specific differences were found.

Differences in the regulation of [3H]idazoxan and [3H]yohimbine binding sites in the rabbit

In vitro studies suggest that [3H]yohimbine binds to alpha 2-adrenoceptors while [3H]idazoxan binds preferentially at a non-adrenergic site. In order to compare in vitro with in vivo effects male New Zealand White rabbits received the following treatments: 5 days idazoxan 1.1 mg/kg per h, 10 days noradrenaline 46 micrograms/kg per h (intravenous infusion), 21 days amitriptyline 30 mg/kg per day (intraperitoneally) or vehicle. The effect of these treatments on the number of [3H]yohimbine and [3H]idazoxan binding sites was examined. Ten days noradrenaline infusion and 21 days amitripytyline treatment significantly reduced [3H]yohimbine binding in kidney and hindbrain membranes respectively, but had no significant effect on [3H]idazoxan binding. Five days idazoxan infusion significantly increased [3H]yohimbine binding in the forebrain, while a significant reduction in [3H]idazoxan binding sites in the kidney was observed. Thus differential regulation of the two binding sites was observed in vivo. These alterations in binding site number are consistent with the differing affinities of noradrenaline and idazoxan for the [3H]yohimbine and [3H]idazoxan binding sites previously observed in vitro and support the hypothesis that in the rabbit idazoxan binds preferentially at non-adrenergic sites while yohimbine binds to an alpha 2-adrenergic site. The idazoxan site may be an imidazoline type of receptor but further work, including functional studies, is required to substantiate this.

Desensitization and down-regulation of brain alpha 2-adrenoceptors by centrally acting antihypertensive drugs

Rabbits were treated with intravenous clonidine (8 mumol kg-1 day-1), guanabenz (20 mumol kg-1 day-1), rilmenidine (80 mumol kg-1 day-1) or vehicle via osmotic minipumps. After 6 days treatment mean arterial pressure (MAP), pressor responses to intravenous alpha-methyl noradrenaline and depressor responses to intracisternal clonidine were studied, and [3H]-yohimbine binding to forebrain and hindbrain examined in vitro. Clonidine, guanabenz and rilmenidine had similar effects on MAP and caused a similar attenuation of the depressor response to intracisternal clonidine, but only guanabenz attenuated pressor responses to intravenous alpha-methyl noradrenaline. Rilmenidine had no effect on [3H]-yohimbine binding to brain membranes. Clonidine treatment decreased binding in hindbrain while guanabenz treatment decreased binding in both fore- and hindbrain. Thus, the depressor effects of chronic treatment did not correlate with the effects on [3H]-yohimbine binding sites in rabbit brain suggesting that the blood pressure lowering effects of many centrally acting antihypertensive drugs are not necessarily dependent on binding to the alpha 2-adrenoceptor site labelled by [3H]-yohimbine.

Idazoxan and brain alpha 2-adrenoceptors in the rabbit

The effect of intravenous infusion of idazoxan on the depressor response to intracisternal clonidine 1 microgram/kg and on [3H]yohimbine binding in the fore- and hindbrain of the rabbit was examined. Idazoxan was infused either acutely (30 min) or chronically (5 days) at doses of 0.56 or 1.1 mg/h. Idazoxan 1.1 mg/h reduced the fall in blood pressure after clonidine. This attenuation of the depressor response was observed in the groups that were given the higher dose of idazoxan both acutely and chronically. The extent of attenuation was not modified by the duration of treatment. The low dose of idazoxan given acutely had no significant effect on the response to clonidine but the chronically infused group showed an enhanced response. A significant increase in the number of [3H]yohimbine binding sites (83%) was observed in the forebrain after 5 days infusion of 1.1 mg/h idazoxan with no change in the hindbrain. The lower dose of infusion did not cause any significant change in [3H]yohimbine binding in either brain region. Thus it appears that the susceptibility of the alpha 2-adrenoceptor binding sites to up-regulation by idazoxan may depend on the brain region observed.

[3H]yohimbine and [3H]idazoxan bind to different sites on rabbit forebrain and kidney membranes

The binding of the alpha 2-adrenoceptor ligands [3H]yohimbine and [3H]idazoxan to rabbit kidney and forebrain membranes was compared. The maximum number of [3H]yohimbine binding sites was higher than the number of [3H]idazoxan binding sites in forebrain and lower in kidney. Large differences were observed in the ability of noradrenaline, adrenaline, idazoxan, rauwolscine, yohimbine and WY 26392 to displace [3H]yohimbine and [3H]idazoxan from their binding sites. These data suggest that [3H]idazoxan and [3H]yohimbine bind to different sites on rabbit tissue membranes.