Attenuated 5-HT2 receptor-mediated responses in hindquarters of diabetic rats

The nitric oxide synthesis/pathway mediates the inhibitory serotoninergic responses of the pressor effect elicited by sympathetic stimulation in diabetic pithed rats

We investigated the involvement of the nitric oxide pathway in the inhibitory mechanisms of 5-hydroxytryptamine (5-HT) in the pressor responses induced by stimulation of sympathetic vasopressor outflow in diabetic pithed rats. Diabetes was induced in male Wistar rats by a single s.c. injection of alloxan. Four weeks later, the animals were anaesthetized, pretreated with atropine, and pithed. Electrical stimulation of the sympathetic outflow from the spinal cord (0.1, 0.5, 1 and 5 Hz) resulted in frequency-dependent increases in blood pressure. The inhibition of electrically induced pressor responses by 5-HT (10 microg/kg/min) in diabetic pithed rats could not be elicited after i.v. treatment with 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10 microg/kg), a guanylyl cyclase inhibitor, or N-omega-L-Arginine methyl ester hydrochloride (L-NAME) (10 mg/kg), a nitric oxide synthase (NOS) inhibitor. The inhibitory effect produced by infusion of the selective 5-HT(1A) receptor agonist 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT) (20 microg/kg/min) was abolished in the presence of ODQ (10 microg/kg), or L-NAME (10 mg/kg) in diabetic pithed rats. The administration of L-Arginine (100 mg/kg) 30 min after L-NAME reproduced the inhibitory effect caused by 5-HT (10 microg/kg/min) and 8-OH-DPAT (20 microg/kg/min) on the electrically induced pressor responses, whereas in the presence of D-Arginine (100 mg/kg)+L-NAME the 5-HT or 8-OH-DPAT inhibitory effect on the pressor responses was abolished. In conclusion, in diabetic pithed rats, the inhibition produced by prejunctional 5-HT(1A) activation on electrically induced sympathetic pressor responses is mediated by the NO synthesis/pathway.

Role of the PKC/CPI-17 pathway in enhanced contractile responses of mesenteric arteries from diabetic rats to alpha-adrenoceptor stimulation

Protein kinase C (PKC) may contribute to enhanced contractile responses of arteries from streptozotocin-diabetic rats to stimulation of G-protein coupled receptors. This was investigated by comparing the effects of PKC inhibitors on contractile responses of mesenteric arteries from diabetic and age-matched control rats to noradrenaline (NA) and endothelin-1 (ET-1). The effects of NA and ET-1 on the distribution of three isoforms of PKC implicated in contraction were also determined. In addition, the effect of NA on phosphorylation of CPI-17, a substrate for PKC, was investigated. Contractile responses of endothelium-denuded arteries from diabetic rats to NA were enhanced, but were normalized by PKC inhibition. In contrast, contractile responses to ET-1 were not significantly different, and were blocked to a similar extent by PKC inhibition, in arteries from control and diabetic rats.NA produced only a small increase in particulate levels of PKCepsilon in control arteries (to 125+/-8% of levels in untreated arteries), but a significant increase in particulate PKCalpha (to 190+/-22%) and a much greater increase in particulate PKCepsilon (to 230+/-19%) in arteries from diabetic rats. ET-1 increased particulate PKCalpha and epsilon to a similar extent in arteries from control and diabetic rats.NA significantly enhanced CPI-17 phosphorylation from a basal level of 22+/-10 to 71+/-7% of total in arteries from diabetic rats, and this was prevented by PKC inhibition. NA had no detectable effect on CPI-17 phosphorylation in arteries from control rats. These data suggest that NA-induced activation of PKC and CPI-17, its downstream target, is selectively enhanced in arteries from diabetic rats, and mediates the enhanced contractile responses to this agonist.

Sarpogrelate: cardiovascular and renal clinical potential

Sarpogrelate is a selective 5-hydroxytryptamine receptor subtype 2A (5-HT2A) antagonist. It is metabolised to racemic M-1 and both enantiomers of M-1 are also antagonists of 5-HT2A receptors. Sarpogrelate inhibits responses to 5-HT mediated by 5-HT2A receptors such as platelet aggregation, vasoconstriction and vascular smooth muscle proliferation. There is no information available on the pharmacokinetics of sarpogrelate. Sarpogrelate is efficacious in animal models of thrombosis, coronary artery spasm, atherosclerosis, restenosis, peripheral vascular disease, pulmonary hypertension, ischaemic heart disease, myocardial infarction, diabetes and kidney disease. Small clinical trials indicate that sarpogrelate may be beneficial in the treatment of coronary artery disease, angina, restenosis, heart valve prostheses surgery, diabetes mellitus, Raynaud's phenomenon, systemic sclerosis and Buerger's disease. Larger, randomised, double-blind, placebo-controlled clinical trials of sarpogrelate in intermittent claudication, coronary artery disease, restenosis and diabetes should be considered.

A role for protein kinase C in the attenuated response to 5-hydroxytryptamine in aortas from streptozotocin-diabetic rats

We investigated protein kinase C participation in the contractile response to 5-hydroxytryptamine (5-HT), and in the interaction between 5-HT and endothelin-1, in aortas from control and diabetic rats. Diabetic rats display attenuated reactivity to 5-HT (i.e., approximately 47% of control maximum). The protein kinase C inhibitor calphostin C (1 microM) significantly reduced responses to 5-HT only in aortas from control rats. In diabetic rats, maximum responses to 5-HT, in the presence of endothelin-1 (3 nM), were not significantly different to controls. The additional presence of calphostin C significantly reduced responses only in aortas from diabetic rats. These results may indicate an abnormality in the protein kinase C second messenger system during diabetes.

Evidence for adrenergic and tachykinin activity in venom of the stonefish (Synanceja trachynis)

The aim of the present study was to investigate previously suggested adrenergic and tachykinin activity, as well as the cardiovascular effects, of venom from the stonefish (Synanceja trachynis). Stonefish venom (60-120 micrograms/kg, i.v.) produced dose-dependent bronchoconstriction in anaesthetised guinea-pigs. This response (100 micrograms/kg, i.v.) was significantly reduced by the neurokinin 1 (NK1) receptor antagonist CP-99,994 (1 mg/kg, i.v.). Contractile responses to venom (4 micrograms/ml) of guinea-pig isolated ileum (GPI) were significantly inhibited by a combination of the sodium channel blocking drug tetrodotoxin (1 microM) and the ganglion blocking drug mecamylamine (10 microM). However, subsequent administration of CP-99,994 (0.1 microM) did not produce further inhibition. Endogenous tachykinin depletion with capsaicin (1 microM) also significantly attenuated responses to venom (4 micrograms/ml) in GPI. Venom (4 micrograms/ml) produced increases in rate and force of contraction of rat spontaneously beating isolated atria which were significantly inhibited by the beta-adrenoceptor antagonist propranolol (5 microM) but not by noradrenergic transmitter depletion with reserpine (4.5 mg/kg, i.p.). In the presence of the alpha 1-adrenoceptor antagonist prazosin (0.3 microM), venom (6 micrograms/ml) significantly inhibited electrically evoked twitches of prostatic segments of rat vas deferens. The inhibitory effect of venom was significantly reduced by the alpha 2-adrenoceptor antagonist idazoxan (1 microM) but not by propranolol (5 microM) or the neurokinin 2 (NK2) receptor antagonist SR-48,968 (0.1 microM). Venom (60-120 micrograms/kg, i.v.) produced dose-dependent increases in mean arterial blood pressure in anaesthetised rats. This pressor response (60 micrograms/kg, i.v.) was significantly reduced by prazosin (10-50 micrograms/kg, i.v.) and the leukotriene receptor antagonist SB205312 (1 mg/kg, i.v.), significantly increased by propranolol (2 mg/kg, i.v.), but not significantly affected by the cyclo-oxygenase inhibitor indomethacin (10 mg/kg, i.v.) or the thromboxane A2/prostaglandin H2 (TP) receptor antagonist GR32191B (1 mg/kg, i.v.). Pressor responses to venom (100 micrograms/kg, i.v.) were also observed in anaesthetised rabbits. These results suggest that stonefish venom contains a component capable of stimulating the release of endogenous tachykinins with subsequent activity at NK1 receptors. The venom also appears to act via stimulation of sodium channels on sensory nerves. The venom also has activity at alpha 2-adrenoceptors and a direct action at beta-adrenoceptors. The effect of venom on blood pressure of anaesthetised rats appears to include a pressor component that is mediated, in part,by alpha-adrenoceptors and leukotriene receptors, and a depressor component that is mediated by beta-adrenoceptors. However, the pressor response does not involve action at TP receptors, or require the production of cyclo-oxygenase metabolites.