Differential inhibition of renin mRNA expression by paricalcitol and calcitriol in C57/BL6 mice.

BACKGROUND/AIMS: Vitamin D receptor activators (VDRAs) may suppress renin expression and VDR-mediated renin inhibitors may offer a novel mechanism to control the RAS. METHODS: We delineated the effects of paricalcitol and calcitriol on PTH, renin, and iCa(2+) in C57/BL6 mice administered vehicle, paricalcitol, or calcitriol (0.01, 0.03, 0.10, 0.33, 1.0 microg/kg s.c.) 3 days/week for 9 days. RESULTS: Paricalcitol produced PTH suppression from 0.03 to 1.0 microg/kg (values between 9.7 +/- 3.3 and 20.7 +/- 4.7 pg/ml; vehicle = 88.0 +/- 16.9) and elicited dose-dependent reductions in renin/GAPDH expression at 0.33 and 1.0 microg/kg (0.037 +/- 0.002, 0.027 +/- 0.003; vehicle = 0.054 +/- 0.003) but produced no increases iCa(2+) at any dose tested. Calcitrol produced PTH suppression at all doses tested (between 6.4 +/- 1.2 and 29.5 +/- 17.2 pg/ml) and renin suppression at 0.10, 0.33, and 1.0 microg/kg (0.029 +/- 0.002, 0.031 +/- 0.003, and 0.038 +/- 0.02). However, at 0.33 and 1.0 mg/kg, calcitriol produced increases iCa(2+) (1.31 +/- 0.03 and 1.48 +/- 0.02 mmol/l; vehicle = 1.23 +/- 0.02 mmol/l). CONCLUSIONS: Paricalcitol produces significant, dose-dependent suppression of renin expression in the absence of hypercalcemia at doses 10-fold above those necessary for PTH suppression. Calcitriol also produced suppression of renin at doses at least 10-fold above those required for PTH suppression, but increases in iCa(2+) were observed at doses only 3-fold above those necessary to elicit renin suppression.

Systemic activation of the calcium sensing receptor produces acute effects on vascular tone and circulatory function in uremic and normal rats: focus on central versus peripheral control of vascular tone and blood pressure by cinacalcet.

Calcium-sensing receptor (CaR) activation decreases serum parathyroid hormone (PTH) and Ca2+ and, despite long-term reductions in mean arterial blood pressure (MAP), may produce acute hypertension in rats, an effect we hypothesized was mediated by constriction of multiple vascular beds. Rats were subjected to 5/6 nephrectomy (NX) or no surgery (Normal); at 7 to 8 weeks, uremia animals were anesthetized and instrumented to record MAP and regional blood flow (carotid, mesenteric, and hindlimb). Cinacalcet [N-(1-naphthalen-1-ylethyl)-3-[3-(trifluoromethyl)phenyl]-propan-1-amine; 1, 3, and 10 mg/kg; 30 min/dose] was infused over 90 min. In NX rats, cinacalcet dose-dependently decreased ionized calcium (iCa2+), elicited a 90% reduction in PTH, and produced dose-dependent self-limiting increases in MAP (from 119 +/- 6 to 129 +/- 5, 142 +/- 4, and 145 +/- 3 mm Hg at the end of each infusion). At 1 mg/kg, carotid vascular resistance (CVR) and mesenteric vascular resistance (MVR) increased to 16 +/- 6 and 18 +/- 6% above baseline, respectively. Hindlimb vascular resistance (HVR) also trended upward (13 +/- 8%). At 3 mg/kg, increases in CVR (38 +/- 10%), MVR (40 +/- 8%), and HVR (39 +/- 14%) were exacerbated; at 10 mg/kg, values remained at or near these levels. The effects of cinacalcet in Normal rats were similar to NX and were attenuated by ganglionic blockade with hexamethonium at low doses but remained significantly elevated at higher doses. Thus, CaR activation acutely increases MAP in uremic and nonuremic rats, responses that occur in parallel to vasoconstriction in multiple vascular beds through both a central and peripheral mechanism of action. Moreover, subsequent mechanistic studies suggest that increases in MAP produced by cinacalcet may be mediated by reduced tonic NO synthase-dependent NO production subsequent to reductions in blood iCa2+.

Role of phosphorus and vitamin D analogs in the pathogenesis of vascular calcification.

Vascular calcification is a mortality risk factor for stage 5 chronic kidney disease patients. We investigated the role of phosphorus and vitamin D analogs in the pathogenesis of vascular calcification using in vivo, ex vivo, and in vitro models. Our results demonstrate that uremic rats receiving a hyperphosphatemia-inducing diet did not exhibit aortic calcification despite elevated levels of serum phosphorus and calcium-phosphorus (CaxP) product. The vitamin D analog 1alpha-hydroxyvitamin-D2 [1alpha(OH)D2] at 0.17 microg/kg raised serum calcium, phosphorus, CaxP product, and aortic calcification in the uremic rats, but 19-nor-1alpha,25(OH)2D2 (19-nor) at the same dose had no significant effect. At 0.67 microg/kg, both 1alpha(OH)D2 and 19-nor had similar effects on serum calcium, phosphorus, and CaxP product, but only 1alpha(OH)D2 induced significant aortic calcification. Only aortic rings from 1alpha(OH)D2-treated uremic rats exhibited a significant increase in 45Ca uptake ex vivo. When aortic rings from normal rats or a primary culture of human coronary artery smooth muscle cells were treated with phosphorus or vitamin D analogs in vitro, high phosphorus induced calcium accumulation and/or 45Ca uptake in a dose- or time-dependent manner, whereas vitamin D analogs including 1alpha(OH)D2 up to 100 nM had no significant effect despite the presence of a functional vitamin D receptor. However, serum from 1alpha(OH)D2-treated uremic rats induced 45Ca uptake into smooth muscle cells cultured in high phosphorus. These results suggest that the regulation of vascular calcification in vivo cannot be easily replicated in the ex vivo or in vitro models, and high phosphorus and some vitamin D analogs such as 1alpha(OH)D2 exert interactive effects on modulating vascular calcification.

Effects of BW245C, a prostaglandin dp receptor agonist, on systemic and regional haemodynamics in the anaesthetized rat.

1. Prostaglandin D (DP) receptor agonists have been shown to induce hypotension in rat models, possibly via peripheral vasodilation. However, it is not known which tissues and organs are most responsive. 2. In the present study, BW245C, a DP receptor-selective agonist, was administered to Inactin (Sigma, St Louis, MO, USA)-anaesthetized rats. Animals received three serial i.v. infusions (17 min each) of either BW245C (escalating doses of 0.3, 3 and 30 microg/kg; n=6) or vehicle (6% ethanol in normal saline; n=6). Mean arterial pressure (MAP) and heart rate were monitored continuously and regional blood flow was determined by the radionuclide-labelled microsphere method at baseline and at the end of each infusion. 3. It was found that BW245C dose-dependently reduced MAP; blood flow increased in forelimb skeletal muscle and skin, resulting in decreases in the regional vascular resistance (RVR) of skeletal muscle to -6+/-13, -53+/-11 and -68+/-6% of baseline following 0.3, 3 and 30 microg/kg BW245C, respectively (P<0.05 vs vehicle treatment for the two higher doses), and skin to -29+/-8, -55+/-8 (P<0.05) and -30+/-16% of baseline, respectively. Relative to vehicle, blood flow and RVR for brain, heart, lung, liver, stomach and kidney were not significantly affected by BW245C. 4. These results demonstrate that the hypotension resulting from DP receptor activation in the rat is mediated primarily through vasodilation of arterioles of skeletal muscle independent of changes in blood flow to vital organs.

Prolactin-induced parental hyperphagia in ring doves: are glucocorticoids involved?

Hyperphagia is a prominent component of the parental behavior repertoire in male and female ring doves and is necessary in order for parents to successfully provision their growing young. Although previous studies implicate both prolactin and the endogenous glucocorticoid, corticosterone, in parental hyperphagia, the functional interactions between these two hormones in regulating changes in feeding activity have not been characterized. These studies examined the possibility that prolactin's orexigenic effects are mediated through the increased secretion of corticosterone. Twice-daily intracerebroventricular (icv) injection of prolactin increased plasma corticosterone concentration in non-breeding doves of both sexes, with males exhibiting more pronounced effects than females. To further test the importance of glucocorticoid signaling in prolactin-induced feeding responses, changes in food intake were investigated in icv prolactin-treated, non-breeding doves following icv infusion of the glucocorticoid receptor antagonist RU38486 or propylene glycol vehicle. No attenuation of prolactin-induced hyperphagia was observed in either sex following co-administration of RU38486 at a dose shown previously to block dexamethasone-induced feeding in doves. These findings suggest that elevated corticosterone titers in blood may contribute to the hyperphagia observed in response to prolactin, but corticosterone signaling through a mammalian-type glucocorticoid receptor is not essential.

Tumor selective antivascular effects of the novel antimitotic compound ABT-751: an in vivo rat regional hemodynamic study.

Selective induction of vascular damage within a growing tumor is a potentially important approach in the search for potent anticancer therapeutics. Tubulin-binding (antimitotic) agents destabilize cellular microtubules, suppress tumor growth, and exert antivascular effects with varying degrees of tumor selectivity in preclinical models. The tumor-selective, antivascular effects of ABT-751, a novel, orally active antimitotic agent, currently in phase II clinical development, were characterized in vivo in the present study. We developed an in vivo rat model designed to quantify acute changes in regional vascular resistance (VR) in both tumor and non-tumor vascular beds simultaneously. Tissue-isolated tumors (1 g) with blood flow supplied by a single epigastric artery were grown in rats. Subsequently, tumor blood flow was measured under anesthesia in solid tumors and also in mesenteric, renal, and normal epigastric arteries. Phenylephrine-induced (1 micromol/kg) increases in VR were not different between tumor and non-tumor epigastric arteries, suggesting that tumor vessels possess relatively normal vasoconstrictive function. ABT-751 (3, 10, and 30 mg/kg; i.v.) produced modest transient increases in mean arterial pressure with no effect on heart rate. Tumor VR increased to 75+/-36, 732+/-172, and 727+/-125% above baseline, respectively (P<0.05 for the 10 and 30 mg/kg doses), whereas VR in normal epigastric arteries was not significantly affected. Administration of ABT-751 produced transient modest ( P<0.05) increases in mesenteric VR and no effect on renal VR. These results demonstrate that ABT-751 produces marked reductions in tumor blood flow in the intact rat at doses that exert negligible effects on normal vascular function.

Glucocorticoids and parental hyperphagia in ring doves (Streptopelia risoria).

These studies explored the possibility that glucocorticoids promote parental care in ring doves by mediating, at least in part, the pronounced increase in food consumption that parent doves exhibit while provisioning their young. Plasma concentrations of the endogenous glucocorticoid corticosterone were found to be significantly higher in breeding females during the posthatching phase than during the incubation period. These differences were not observed in male breeding partners, but sex differences in daily activity rhythms are well documented in breeding doves, and blood sampling at different times of day would be required to adequately characterize the pattern of corticosterone in males during these breeding stages. In studies on nonbreeding doves, twice-daily intracerebroventricular (icv) injections of the synthetic glucocorticoid dexamethasone (DEX) increased food intake by 25-50% in both sexes, and further studies in males revealed that the increase was directly related to the dose of DEX administered. The highest dose of DEX given icv (1.0 microg/day) was not effective in stimulating feeding when given systemically, thereby suggesting that the hyperphagic action of DEX is exerted directly on the central nervous system. The icv infusion of the selective glucocorticoid receptor antagonist RU38486 blocked the hyperphagic effects of twice-daily icv injections of DEX in both sexes. Collectively, these data support the hypothesis that corticosterone contributes to the parental hyperphagia exhibited by breeding doves during the posthatching period. They also suggest that these orexigenic effects are mediated in part by CNS binding sites that resemble mammalian glucocorticoid receptors.