GSK256073, a selective agonist of G-protein coupled receptor 109A (GPR109A) reduces serum glucose in subjects with type 2 diabetes mellitus.

AIMS: This clinical trial assessed whether a potent, selective GPR109A agonist, GSK256073, could, through inhibition of lipolysis, acutely improve glucose homeostasis in subjects with type 2 diabetes mellitus. METHODS: Thirty-nine diabetic subjects were enrolled in the randomized, single-blind, placebo-controlled, three-period crossover trial. Each subject received placebo and two of four regimens of GSK256073 for 2 days. GSK256073 was dosed 5 mg every 12 h before breakfast and supper (BID), 10 mg every 24 h before breakfast (QD), 25 mg BID and 50 mg QD. RESULTS: The change from baseline weighted mean glucose concentration for an interval from 24 to 48 h after the initial drug dose was significantly reduced for all GSK256073 regimens, reaching a maximum of -0.87 mmol/l (-1.20, -0.52) with the 25 mg BID dose. Sustained suppression of non-esterified fatty acid (NEFA) and glycerol concentrations was observed with all GSK256073 doses throughout the 48-h dosing period. Serum insulin and C-peptide concentrations fell in concert with glucose concentrations and calculated HOMA-IR scores decreased 27-47%, consistent with insulin sensitization. No marked differences were evident between either 10 and 50 mg total daily doses or QD versus BID dosing. CONCLUSIONS: Administration of a GPR109A agonist for 2 days significantly decreased serum NEFA and glucose concentrations in diabetic subjects. Glucose improvements were associated with decreased insulin concentrations and measures of enhanced insulin sensitivity. Improved glucose control occurred with GSK256073 doses that were generally safe and not associated with events of flushing or gastrointestinal disturbances.

Effect of DMPPO, a phosphodiesterase type 5 inhibitor, on hypoxic pulmonary hypertension in rats.

1. Cyclic guanosine 3'-5'-monophosphate (cyclic GMP) is the second messenger of important physiologically active mediators controlling the pulmonary vascular tone. To potentiate the effects of cyclic GMP on the pulmonary vasculature, we used DMPPO, a new selective PDE-5 inhibitor, and examined its action in a rat model of hypoxic pulmonary hypertension. 2. Levels of cyclic GMP measured during baseline conditions at 5 and 60 min of perfusion were similar in the perfusate of isolated lungs from normoxic and chronically hypoxic rats and did not differ with time. Pretreatment with DMPPO (1 microM) induced a larger increase in cyclic GMP concentration in the perfusate from chronically hypoxic rat lungs (31+/-36 at 5 min to 1821+/-83 pmol ml(-1) at 60 min) than in normoxic rat lungs (329+/-20 to 1281+/-127 pmol ml(-1), P<0.05). 3. In isolated lungs preconstricted with U-46619, pretreatment with DMPPO (1 microM) potentiated the vasodilator effects of atrial natriuretic peptide (100 pM-10 nM) and sodium nitroprusside (1 pM 10 nM), but did not alter vasodilation to isoproterenol. 4. In conscious rats previously exposed to 15 days hypoxia and studied under 10% O2, DMPPO (0.01, 0.05 and 0.1 mg kg(-1), i.v. bolus) caused a dose-dependent decrease in pulmonary arterial pressure (Pap) with no change in systemic artery pressure (Sap) and cardiac output. 5. Continuous infusion of DMPPO (0.1 mg kg(-1) h(-1) i.v. by osmotic pumps) in rats exposed to 10% O2 during 2-weeks reduced the Pap (P<0.05) and the degree of muscularization of pulmonary vessels at the alveolar wall (P<0.01) and alveolar duct levels (P<0.05) despite no significant change in right ventricular hypertrophy. 6. These results suggest that cyclic GMP phosphodiesterase inhibition may selectively dilate pulmonary circulation during chronic hypoxia.

Cardiovascular effects of a novel, potent and selective phosphodiesterase 5 inhibitor, DMPPO: in vitro and in vivo characterization.

1. The aim of this study was to investigate the cardiovascular effects of a novel, potent and specific phosphodiesterase 5 (PDE 5) inhibitor, 1,3 dimethyl-6-(2-propoxy-5-methane sulphonylamidophenyl)-pyrazolo[3,4-d]pyrimidin-4-(5H)-one (DMPPO) in phenylephrine-precontracted rat aortic rings and different in vivo rat preparations. 2. DMPPO elicited a concentration-dependent relaxation of rat aortic rings with functional endothelium. DMPPO-induced relaxation was abolished by endothelium removal or pretreatment with the soluble guanylate cyclase inhibitor, methylene blue (10 microM). 3. In aortic rings without endothelium, the potency (pD2= -log10 EC50) of atrial natriuretic peptide (ANP) to induce relaxation increased from 8.13 +/- 0.05 in the absence of DMPPO, to 8.32 +/- 0.05 and 8.52 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. Similarly, the potency of sodium nitroprusside (SNP) in inducing relaxation increased from 7.38 +/- 0.07 in the absence of the PDE 5 inhibitor to 8.07 +/- 0.11 and 8.15 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. In contrast, relaxation to the adenylate cyclase activator, forskolin, was unchanged by DMPPO (100 nM). 4. In rings without endothelium, DMPPO (100 nM) increased by 2.5 fold intracellular levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). Moreover, DMPPO (100 nM) potentiated the increases in cyclic GMP levels induced by ANP (30 nM) by 3 fold and SNP (30 nM) by 2.7 fold. Adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels were not modified by DMPPO. 5. In anaesthetized normotensive or spontaneously hypertensive rats (SHR), DMPPO (2 and 5 mg kg-1, i.v.) lowered blood pressure without affecting heart rate. Similarly, in conscious SHR, orally administered DMPPO (5 mg kg-1) induced a 25 mmHg decrease in blood pressure for at least 7 h without modifying heart rate. Meanwhile, urinary cyclic GMP was increased by 50% whereas cyclic AMP remained unchanged. 6. In normotensive anaesthetized rats, sodium nitroprusside (SNP) (i.v. bolus) induced a decrease in blood pressure which rapidly returned to baseline. In DMPPO (1 mg kg-1, i.v.)-treated rats, the hypotensive effects of SNP (10 to 100 micrograms kg-1) were prolonged over time whereas the peak effect was unchanged. 7. In pithed rats, phenylephrine (i.v. bolus) induced dose-dependent increases in blood pressure. Pretreatment with DMPPO (5 mg kg-1, i.v.) partially inhibited the pressor response to phenylephrine (0.3 to 100 micrograms kg-1). 8. In conclusion, the potent and selective PDE 5 inhibitor, DMPPO, produces relaxation in isolated vessels in the presence of a cyclic GMP drive and reduces blood pressure of intact animals. Its high oral bioavailability and long duration of action should make it a useful tool to study the role of cyclic GMP in various biological systems.

Differential inhibition of the pressor effects of natural pro-endothelins by phosphoramidon in rats.

The vasopressor responses of three natural endothelins (ET-1, ET-2, ET-3) and their precursors, pro-ETs, were studied in a pithed rat preparation, which allows the profile and the potency of vasoconstrictor agents to be determined in the absence of central control of the cardiovascular system. ET-1 was found to be 4- and 8-fold more potent in raising blood pressure than ET-2 and ET-3, respectively. The immediate precursors of these isopeptides, h-pro-ET-1 (human), p-pro-ET-1 (porcine), pro-ET-2 and pro-ET-3, produced significantly smaller pressor responses than their respective ETs, when measured either as peaks or as areas under the time-effect curve. Hence, the bioavailability of h-pro-ET-1, p-pro-ET-1 and pro-ET-2, assessed on the basis of these two parameters, was approximately 50% of that of their corresponding ET, whereas the bioavailability of pro-ET-3 was only 25% that of ET-3. Phosphoramidon inhibits metallopeptidases, the enzymes that convert pro-ETs to ETs. The approximate i.v. doses of phosphoramidon reducing by 50% the pressor effects of the pro-ETs were 2.5, 0.625, less than 2.5 (this dose produced 75% inhibition) and 5 mg/kg i.v. for h-pro-ET-1, p-pro-ET-1, pro-ET-2 and pro-ET-3, respectively. In conclusion, these results indicate that the rat may have more than one pro-ET converting enzyme, each specific for one of the natural pro-ETs studied, although the alternative explanation, that there is a single enzyme with different affinities for these pro-ETs, cannot be entirely dismissed.

Cross tachyphylaxis to endothelin isopeptide-induced hypotension: a phenomenon not seen with proendothelin.

1. In anaesthetized rats, an i.v. injection of endothelin-1 (0.25 nmol kg-1) evoked a rapidly appearing (maximal effect within 15 s) and short lasting (3 min) fall in blood pressure with tachyphylaxis occurring so that it was reduced by 50% by the last of 4 injections given 10 min apart. This property was also shared by endothelin-2, endothelin-3 and vasoactive intestinal contractor (VIC). 2. Cross tachyphylaxis between the isopeptides occurred. However, under the same experimental conditions the hypotensive effects of acetylcholine, adenosine, atrial natriuretic peptide (ANP) and substance P were reproducible and not modified in animals in which endothelin-1 no longer lowered blood pressure. Thus, the mechanism of the hypotensive action of endothelin peptides is different from that of acetylcholine, adenosine, ANP, and substance P. 3. In pithed rats, endothelin-1 (0.25 nmol kg-1) and its precursor human proendothelin (h-proendothelin) (0.5 nmol kg-1) induced pressor responses of a similar magnitude, which for h-proendothelin (up to 5.0 nmol kg-1) were not preceded by a hypotensive phase. The pressor effects of endothelin-1, like those of vasopressin, were reproducible upon repeated i.v. injections. 4. Rats given a 10 min infusion (0.1 nmol kg-1 min-1) of endothelin-1 showed no hypotensive response to an i.v. bolus injection of endothelin-1, whereas animals pretreated with an equipressor infusion of h-proendothelin did not develop tachyphylaxis to endothelin-1. 5. In pitched rats, endothelin-1, at a dose inducing the same maximal increase in blood pressure as h-proendothelin, was approximately 3 fold more potent as a mesenteric vasoconstrictor than h-proendothelin. These results suggest that if h-proendothelin is processed to endothelin-1, this transformation is not uniform throughout the vascular system. 6. The pressor response of h-proendothelin in pithed rats was dose-dependently inhibited by phosphoramidon (2.5-5.0mgkg '). However, this compound did not antagonize the effects of endothelin-1(0.25 nmol kg- ) or those of h-proendothelin (0.5 nmol kg- ) once developed. 7. Although some of these results may suggest that h-proendothelin does not undergo in vivo conversion to endothelin-1, the results obtained with phosphoramidon suggest that h-proendothelin is converted into endothelin-1. Therefore, the amount of endothelin-1 so produced can elicit pressor responses or regional vasoconstriction, but is insufficient to lower blood pressure and to inhibit endothelin-1-induced hypotension. 8. The mechanism of the tachyphylaxis does not appear to be depletion of endothelium-derived relaxing factor, since agents coupled to the latter endogenous vasorelaxant substance do not exhibit crosstachyphylaxis with endothelin-1. It is suggested that upon repeated or sustained exposure to endothelin-1, the endothelin-1 receptors mediating hypotension decrease in number and/or undergo conformational changes making them refractory to activation. Alternatively, the depletion of a blood-borne agent responsible for the hypotension could be involved.

SK&F 87516, a close analog of fenoldopam, is a partial agonist at dopamine-1 and alpha-2 receptors and produces stimulation of 5-hydroxytryptamine-2 receptors in the cardiovascular system of the rat.

In pentobarbital-anesthetized rats, SK&F 87516 (1.25-80 micrograms/kg/min intravenously over 15 min), the fluoro analog of the selective DA-1 dopamine receptor agonist fenoldopam produced dose-related decreases in carotid artery blood pressure that faded during the infusion period. These effects were abolished by SCH 23390, prolonged by ritanserin, but unchanged by bilateral vagotomy, atenolol, ICI 118,551, idazoxan, methylatropine or S-sulpiride. SK&F 87516 also inhibited the hypotensive effects of clonidine and of the DA-1 receptor agonist fenoldopam, but not of acetylcholine. In pithed rats, SK&F 87516 produced a biphasic vasopressor response. The initial phase was enhanced by SCH 23390 and converted to a transient hypotension by idazoxan. The secondary response was inhibited by ritanserin and enalapril. In pithed, but not in intact rats, SK&F 87516 increased plasma renin activity. In intact rats, SK&F 87516 produced dose-related bradycardic effects that were inhibited (50%) by idazoxan, methylatropine or bilateral vagotomy and abolished by chlorisondamine or pithing. In pithed rats pretreated with either saline or idazoxan, SK&F 87516 reduced the tachycardia to electrical stimulation of preganglionic more than that to postganglionic cardioaccelerator nerve fibers. However, it did not modify heart rate increases evoked by intravenous norepinephrine. In conclusion, SK&F 87516 produces hypotension via vascular DA-1 receptor stimulation. The fading of this effect during the infusion of SK&F 87516 may be related to the partial agonist property of this compound at DA-1 receptors and the stimulation of 5-hydroxytryptamine-2 receptors. SK&F 87516 also behaves as a partial agonist at alpha-2 adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential pharmacological profile of endothelin-1 and its precursor, big endothelin.

In pentobarbital-anesthetized rats endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3), and mouse ET-2 (mET-2), in contrast to human big ET-1 (h-big ET), administered as i.v. bolus injections (0.25 nmol/kg i.v.) produced rapidly appearing and short-lasting blood pressure decreases. This effect was markedly inhibited (80-100%) after an 8-min i.v. infusion (0.1 nmol/kg/min over 10 min) of any of the ET studied, but not by h-big ET, the precursor of ET-1. Similarly, in pithed rats given a 10 min i.v. infusion of an equipressor dose (0.1 nmol/kg/min) of ET-1 or h-big ET, the hypotensive effects of ET-1 were entirely blocked only in the group of animals pretreated with ET-1. In pithed rats, ET-1 (0.25 nmol/kg i.v.) and h-big ET (0.5 nmol/kg i.v.) produced equivalent maximal pressor responses and the same pattern of increase in systemic, hindquarter, and renal vascular resistance. However, ET-1 was three times more potent than h-big ET as a vasoconstrictor of the mesenteric bed. Also the pressor response to h-big ET, but not ET-1 (0.25 nmol/kg i.v.), was markedly inhibited by the metalloprotease inhibitor phosphoramidon (5 mg/kg i.v.). These results indicate that the hypotensive effects of ET isopeptides have a common mechanism because they elicit cross tachyphylaxis, although h-big ET did not inhibit the decrease in blood pressure produced by ET-1. A possible explanation for this finding is that h-big ET has intrinsic pressor activity but does not have affinity for receptors mediating the vasodepressor effects of ET isoforms. Alternatively, h-big ET is converted into ET-1 too slowly to yield biophase concentrations of ET-1 necessary for lowering blood pressure and developing tachyphylaxis to ET-isoform-induced hypotension. Finally, if the pressor effects of h-big ET are mediated by ET-1 formation, phosphoramidon can be considered as an inhibitor of the endothelin-converting enzyme.

Vasodilator activity of endothelin-1 and endothelin-3: rapid development of cross-tachyphylaxis and dependence on the rate of endothelin administration.

In pentobarbital anesthetized cats, i.v. bolus injections of endothelin-1 (ET-1, 1 microgram) and ET-3 (3 micrograms) produced a rapidly appearing but short-lasting fall in aortic blood pressure followed in the case of ET-1 only by a small pressor response. When these peptides were administered repeatedly after 10- to 12-min intervals, there was a gradual attenuation of the hypotension that by the fourth injection was replaced by a monophasic pressor response. The i.v. infusion of ET-1 (0.3 microgram/min) or ET-3 (0.9 microgram/min) for 20 min produced sole systemic vasoconstriction. The decrease in blood pressure produced by an i.v. bolus injection of ET-1 and ET-3 was no longer observed 5 min after the end of the ET-1 or ET-3 infusion. In contrast, the hypotensive activity of bradykinin was not modified after the depressor responses to ET-1 and ET-3 had disappeared. Thus, the failure of i.v. bolus injections of ET-1 and ET-3 to lower blood pressure under these experimental conditions cannot be attributed to the development of tachyphylaxis to endogenous endothelium-derived relaxant factor, which is known to mediate the effects of bradykinin. These results suggest that ET-1 and ET-3 share a single vascular receptor for vasodilation, which becomes refractory upon repeated or maintained exposure to these peptides. Alternatively, this refractoriness may be due to depletion of an intracellular mediator(s) that is jointly used by the membrane binding sites of ET-1 and ET-3. Moreover, the present data suggest that the vasodilator activity of ETs depends on the rate of the peptide administration.

Hemodynamic and pharmacological evaluation of the vasodilator and vasoconstrictor effects of endothelin-1 in rats.

In awake normotensive and spontaneously hypertensive rats as well as pentobarbital-anesthetized normotensive rats, endothelin-1 (ET-1, 0.063-0.5 nmol/kg i.v.) produced rapidly appearing, transient, dose-related falls in mean carotid artery blood pressure followed by slowly developing small pressor responses. In the latter preparation, the hypotension was due to a decrease in systemic vascular resistance inasmuch as cardiac output increased slightly. Bilateral vagotomy, BW 755c, glibenclamide, idazoxan, propranolol, methylatropine, methysergide or promethazine pretreatment failed to modify the hypotension induced by ET-1 (0.25 nmol/kg i.v.), but this effect was blocked entirely when ET-1 was injected 8 min after starting an i.v. infusion of ET-1 (0.1 nmol/kg/min for 10 min). In pithed rats, ET-1 (0.125-1.0 nmol/kg i.v.) produced sustained pressor responses which were accompanied by reductions in cardiac output. This peptide (0.25 nmol/kg i.v.) did not affect renal vascular resistance significantly but increased (200%) mesenteric resistance substantially more (3-fold) than systemic or hindquarter resistance. The pressor effects of ET-1 were reduced by diltiazem, nitrendipine, verapamil or cromakalim and unchanged after BW 755c, desipramine, enalapril, indomethacin, methysergide, phentolamine or SK&F 100273. The sustained pressor response evoked by an i.v. infusion of ET-1 (0.25 nmol/kg/min/60 min) was also antagonized markedly by nitrendipine and cromakalim. In pithed rats with vasopressin-supported blood pressure, ET-1 produced a short-lasting hypotension which faded entirely after three successive injections of the peptide. Finally, ET-1 (0.4-0.8 nM) evoked greater contractile responses in rat aortic rings deprived of a functional endothelium than in intact preparations. However, in the latter preparation precontracted with norepinephrine, ET-1, in contrast to acetylcholine, failed to evoke vasorelaxation. In aortic rings, the sustained contractile effects of ET-1 (3.2 nM) were reduced moderately by nitrendipine (50 nM) and markedly by cromakalim (0.8 microM). In contrast, the latter compounds antagonized strongly the contractile response to KCl (25 mM). In conclusion, ET-1 appears to produce active vasorelaxation and vasoconstriction via stimulation of specific receptors on blood vessels. The tolerance to the hypotensive effect of ET-1 may indicate that either the receptor site for ET-1 becomes refractory or, alternatively, it is coupled to easily depletable endogenous hypotensive mediators. Finally, inasmuch as the vasoconstrictor effects of ET-1 can be easily counteracted by calcium antagonists under in vivo but not in vitro conditions, the membrane coupling mechanism for ET-1 may not be exactly the same in conductance or resistance vessels.

Endothelin--a new family of endothelium-derived peptides with widespread biological properties.

Endothelin (ET) is a novel family of three isopeptides (ET-1, ET-2, ET-3) each containing twenty-one amino acids and two disulfide bonds. Initially isolated from the supernatant of cultured porcine aortic endothelial cells, ET is stored as a preproform and released through an unusual proteolytic cleavage. In general, ET-1, ET-2, ET-3 differ quantitatively but not qualitatively in their biologic activity. ET have potent contractile activity in a variety of isolated tissues including arteries veins, trachea, duodenum urinary bladder and uterus. In vivo, ET possesses potent vasodilator and vasoconstrictor properties. Although the mechanisms mediating the hemodynamic effects of ET are not entirely clarified, recent evidence indicates a role for endothelium-derived relaxant factor (EDRF), protein kinase C and extracellular calcium. Moreover, ET appears to produce inflammation and bronchoconstriction through the formation of arachidonic acid metabolites via the cyclooxygenase pathway. The presence of ET binding sites in blood vessels and in several organ systems suggests ET may have important regulatory functions, which remain to be determined.