Vascular pharmacodynamics of NG-nitro-L-arginine methyl ester in vitro and in vivo

The inhibitory effects of NG-nitro-L-arginine methyl ester (L-NAME) on endothelium-dependent vasodilatation were studied in conscious rats and isolated rat aortic rings. In phenylephrine (PE, ED90)-preconstricted aortae, L-NAME caused prolonged and complete inhibition of acetylcholine (ACh)-induced relaxation with IC50 of 4 x 10(-7) M and Hill coefficient (n) of 1. The inhibition was abolished by L-arginine (L-Arg), independently of whether it was applied 10 min earlier or 4 hr later than L-NAME. Intravenous bolus injection of L-NAME caused prolonged increases in mean arterial pressure (MAP), with Emax of 50 +/- 7 mm Hg, ED50 of 5 +/- 1 x 10(-6) mol/kg and n of 2. Intravenous infusion of L-Arg shifted the dose-MAP curve of L-NAME to the right without changing Emax or n. A modified Schild plot (n = 2) for the action of L-NAME gave a slope not different from unity, suggesting that L-Arg inhibits competitively the MAP response of L-NAME. Intravenous infusion of ACh decreased MAP in rats treated with L-NAME (4.8 x 10(-5) mol/kg) or PE. Compared to PE-treated rats, L-NAME inhibited the depressor response to ACh by 50%. Thus, a dose of L-NAME 10 times its ED50 in raising MAP only partially blocked the depressor responses to ACh.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex difference in the antidiuretic activity of vasopressin in the rat

A possible gender difference in the antidiuretic activity of vasopressin was studied in male and female Sprague-Dawley rats. Infusion of vasopressin (3-100 pg.kg-1.min) into conscious, chronically instrumented water-loaded rats resulted in a dose-dependent antidiuresis in both male and female rats. Male rats, however, were more than three times more sensitive to vasopressin than female rats. Thus the effective doses of vasopressin (pg.kg-1.min-1) to decrease urine flow to 30 microliters.min-1.100 g-1 (18 +/- 5 in males; 58 +/- 12 in females), to increase urine osmolality to 600 mosmol/kgH2O (35 +/- 5 in males; 119 +/- 15 in females), and to decrease free water clearance to 30 microliters.min-1.100 g-1 (8 +/- 3 in males; 28 +/- 7 in females) were significantly (P < 0.05) lower in males. Furthermore, in vitro studies in papillary collecting duct cells demonstrated a significantly higher density of vasopressin V2 receptors and a greater ability of vasopressin to stimulate adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in males than in females. Vasopressin V2-receptor density (maximum binding) was 359 +/- 47 and 238 +/- 22 fmol/mg in male and female rats, respectively (P < 0.05). There was no difference in apparent dissociation constants (Kd). Vasopressin resulted in a dose-dependent increase in cAMP accumulation in papillary collecting duct cells, and at the highest concentration of vasopressin used (10(-8) M) cAMP increased from 44 +/- 10 to 182 +/- 51 fmol/micrograms protein in males and from 30 +/- 4 to 91 +/- 18 fmol/micrograms protein in females (P < 0.05). (ABSTRACT TRUNCATED AT 250 WORDS)

Specific ammonium ion requirement for functional ribosomal RNA tertiary structure

In compactly folded RNAs, coordination or hydrogen bonding of cations in specific sites is a potentially important aspect of the tertiary structure. NH4+ specifically stabilizes the tertiary structure of a conserved, 58-nt fragment of the large subunit ribosomal RNA, as judged in two ways: a melting transition associated with tertiary interactions is sharpened and stabilized more effectively by NH4+ than by any alkali metal cation, and the affinity of the RNA fragment for ribosomal protein L11 or the antibiotic thiostrepton is approximately 10-fold stronger when measured in NH4+ than in Na+. The dependence of the melting temperature on NH4+ concentration shows that a single bound ion is responsible for these effects. The requirement of different ribosome functions for NH4+ suggests that other such sites exist in ribosomal RNAs.

Recent cardiovascular drugs from Chinese medicinal plants

This paper describes the pharmacology of some recent cardiovascular drugs derived from plants used in Chinese traditional medicine. The groups of compounds discussed are benzylisoquinolines (several), tetrahydropyrazine (also called ligustrazine), rhynchophylline and hirsutine, ginkgolides and other PAF inhibitors, coumarins, and ginsenosides, plus a miscellaneous group; approximately 30 substances in all. The plant sources and the pharmacology are indicated for the drugs in each group. By far the most studied compounds are the benzylisoquinolines, especially tetrandrine. The types of pharmacological activity recently described for cardiovascular drugs from plants include calcium antagonism, adrenoceptor antagonism, antagonism of platelet activating factor (PAF), and the ability to act as antioxidants. Hundreds of chemicals have been isolated and identified as constituents of thousands of plants but the basic and clinical pharmacology is known for only a handful of these drugs. Much more research is needed, especially with regard to the pharmacology, both basic and clinical, of the pure chemicals derived from plants.

Inhibitory actions of diphenyleneiodonium on endothelium-dependent vasodilatations in vitro and in vivo

1. This study examined the in vitro and in vivo inhibitory effects of diphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synthase, on endothelium-dependent vasodilatations. 2. DPI (3 x 10(-8)-3 x 10(-6) M) concentration-dependently inhibited acetylcholine (ACh)-induced relaxation in preconstricted rat thoracic aortic rings, with an IC50 of 1.8 x 10(-7) M and a maximal inhibition of nearly 100%. DPI (3 x 10(-6) M) also completely inhibited the relaxation induced by the calcium ionophore, A23187 but not by sodium nitroprusside (SNP). The inhibitory effect of DPI (3 x 10(-7) M) on ACh-induced relaxation was prevented by pretreatment with NADPH (5 x 10(-3) M) and FAD (5 x 10(-4) M) but not L-arginine (L-Arg, 2 x 10(-3) M). Pretreatment with NADPH did not alter the inhibitory effect of NG-nitro-L-arginine on ACh-induced relaxation. 3. The inhibitory effect of DPI on ACh-induced relaxation in the aortae lasted > 4 h after washout. In contrast to pretreatment, post-treatment (1 h later) with NADPH (5 x 10(-3) M) reversed only slightly the inhibitory effect of DPI. 4. In conscious rats, DPI (10(-5) mol kg-1) inhibited the depressor response to i.v. infused ACh, but not SNP. However, it caused only a transient pressor response which was previously shown to be due completely to sympathetic activation. 5. Thus, DPI is an efficacious and 'irreversible' inhibitor of endothelium-dependent vasodilatation in vivo and in vitro. The mechanism of the inhibition may involve antagonism of the effects of FAD and NADPH, co-factors of NO synthase. However, unlike the N0-substituted arginine analogues (another class of NO synthase inhibitors), DPI-induced suppression of endothelium-dependent vasodilatation in vivo does not lead to a sustained rise in blood pressure.

Selective inhibition of pressor and haemodynamic effects of NG-nitro-L-arginine by halothane

We investigated the characteristics of inhibition by halothane of the pressor responses to NG-substituted L-arginine derivatives, nitric oxide (NO) synthase inhibitors. Intravenous (i.v.) bolus injections of NG-nitro-L-arginine (L-NNA, 1-32 mg/kg), NG-nitro-L-arginine methyl ester (L-NAME, 0.4-12.8 mg/kg), norepinephrine (NE, 0.25-8 micrograms/kg) and angiotensin II (AII, 0.02-0.64 micrograms/kg) each caused dose-dependent pressor responses in conscious rats. Halothane attenuated responses to the highest dose of NE and AII by approximately 18% but completely abolished responses to L-NNA and L-NAME. The haemodynamic effects of L-NNA were further examined by the microsphere technique in two groups of conscious rats and two groups of halothane-anaesthetized rats. An i.v. bolus injection of L-NNA (16 mg/kg) in conscious rats increased mean arterial pressure (MAP) and total peripheral resistance (TPR) and reduced heart rate (HR) and cardiac output (CO). These changes were associated with reduced conductance in all vascular beds, with the greatest reduction in the lungs and the least in the liver. In halothane-anaesthetized rats, L-NNA caused significant but markedly less change in MAP, HR, TPR, and CO as compared with those in conscious rats. The vasoconstrictor effects of L-NNA were attenuated by halothane in all beds except liver and spleen, with the greatest inhibition in heart. Our results suggest that NO plays a role in maintenance of peripheral vascular resistance and that halothane selectively and "noncompetitively" inhibits the vasoconstrictor effects of NO synthase inhibitors.

Effect of dietary protein on the renal response to fenoldopam in diabetic rats

The effect of the dopamine D1 receptor agonist, fenoldopam, was studied in streptozotocin-induced diabetic rats treated with insulin to maintain a moderate hyperglycemia and fed a low, normal or high protein diet. Fenoldopam at 1 microgram/kg per min i.v. resulted in a significant increase in both glomerular filtration rate (GFR) and renal blood flow (RBF) in diabetic rats fed a normal protein diet. In rats fed a low protein diet, fenoldopam failed to alter either parameter, however, there was a very significant increase in both GFR and RBF in diabetic rats fed a high protein diet. Since diabetes is associated with a decrease in both urinary dopamine excretion as well as the hyperemic response to protein ingestion, it is possible that stimulation of dopamine D1 receptors by fenoldopam restores renal functional reserve in diabetic animals. The observation that the dopamine D1 receptor antagonist, SCH 23390, at a dose (1 microgram/kg per min) that abolished the renal vasodilator effects of fenoldopam, failed to alter renal hemodynamics in diabetic rats suggests that endogenous dopamine has little effect.

Halothane inhibits the pressor effect of diphenyleneiodonium

1. We have recently found that diphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synthase, causes pressor and tachycardic responses in pentobarbitone- but not halothane-anaesthetized rats. The present study investigated the mechanism by which halothane suppresses the pressor response of DPI. The effects of halothane on the pressor response of DPI were also compared with those of other anaesthetic agents. 2. In conscious rats, i.v. bolus injections of DPI (0.025- 1.6 mg kg-1) caused dose-dependent increases in mean arterial pressure (MAP), with ED90 of 0.07 +/- 0.01 mg kg-1 and maximal rise of MAP (Emax) of 59 +/- 2 mmHg. While ketamine potentiated Emax without altering the ED50 and pentobarbitone increased the ED50 without changing Emax of the pressor response to DPI, chloralose, urethane and ethanol displaced the curve to the right and potentiated Emax. In contrast, halothane (0.5-1.25%) dose-dependently and non-competitively reduced the pressor responses to DPI. 3. Intravenous bolus injection of a single dose of DPI (1.6 mg kg-1) caused immediate and large increases in plasma noradrenaline and adrenaline, as well as MAP in conscious rats. Halothane (1.25%) almost completely inhibited these increases. 4. The results suggest that DPI causes a pressor response in conscious rats by activating the sympathetic nervous system and halothane abolishes this pressor response by inhibiting activities of the sympathetic nervous system. The results also show that influences of anaesthetics must be taken into consideration when evaluating pressor response of vasoactive agents.

Comparison between carvedilol and captopril in rats with partial ablation-induced chronic renal failure

1. The effect of the novel beta-adrenoceptor antagonist and vasodilator, carvedilol (SK&F 105517, approximately 70 mg kg-1 daily in the food), and captopril (approximately 38 mg kg-1 daily in the drinking fluid) on the progression of chronic renal failure in rats was studied. 2. Six weeks following partial renal ablation, the urinary protein excretion of the carvediol- (60 +/- 21 mg day-1) and captopril-treated (35 +/- 9 mg day-1) animals was less than 50% that of control rats (133 +/- 27 mg d-1). 3. Serum creatinine (Scr) and urea nitrogen (SUN) concentrations of the carvedilol-(Scr, 0.63 +/- 0.09 mg dl-1; SUN, 11.3 +/- 1.2 mg dl-1) and captopril-treated (Scr, 0.82 +/- 0.05 mg dl-1; SUN, 14.1 +/- 1.5 mg dl-1) animals were also significantly (P < 0.05) lower than that observed in control animals (Scr, 1.4 +/- 0.3 mg dl-1; SUN, 19.2 +/- 3.9 mg dl-1), indicating that glomerular filtration rate was improved by both drugs. Plasma renin activity was significantly (P < 0.05) higher in captopril-treated rats (24.7 +/- 4.6 ng angiotensin I ml-1 h-1) than in either carvedilol-treated (7.9 +/- 1.4 ng angiotensin I ml-1 h-1) or control animals (7.4 +/- 1.0 ng angiotensin I ml-1 h-1). 4. Histological examination of the kidneys demonstrated a significantly reduced glomerular hypertrophy and glomerulosclerosis in those animals receiving carvedilol or captopril compared to controls. 5. Serum carvedilol concentration measured every 6 h for 24 h was variable and ranged on average from 57 +/- 13 ng ml-1 at 16 h 00 min to 121 +/- 31 ng ml-1 at 03 h 00 min. These data indicate that the rats probably had 24 h systemic exposure to carvedilol.6. The present study indicates that carvedilol is effective in attenuating the progression of chronic renal failure in rats.

Attenuated glomerular cGMP production and renal vasodilation in streptozotocin-induced diabetic rats

Because diabetes is associated with impaired vascular endothelium, we have investigated endothelium-dependent cGMP stimulation in isolated glomeruli and renal vasodilation in normal and diabetes mellitus (DM) rats. Rats treated with streptozotocin (60 mg/kg iv) developed high blood glucose, polyuria, enlarged kidneys, and slow weight gain compared with control animals. Chronic treatment with insulin reversed these changes. In isolated glomeruli, the endothelium-dependent vasodilator, acetylcholine (ACh), stimulated cGMP accumulation concentration dependently; however, the response was significantly attenuated in glomeruli from DM rats when compared with normal rats or DM rats treated with insulin. Sodium nitroprusside-induced cGMP accumulation was also slightly but significantly reduced in glomeruli from DM rats, however, the response to atriopeptin III was unaltered. In rats, intravenous infusion of ACh (1 and 10 micrograms.kg-1.min-1) moderately decreased blood pressure and increased renal blood flow without a significant change in glomerular filtration rate. The renal vasodilatory response to ACh was significantly diminished in DM rats, but not in DM rats treated with insulin. Acute treatment with insulin did not restore the ACh response, although the blood glucose level was normalized. We conclude that there is a reduced renal vasodilatory response observed in DM, and this is due to an impairment of the renal vascular endothelium to produce endothelium-dependent relaxation factor (nitric oxide) and/or a defective soluble guanylate cyclase.

Purification of aspartate aminotransferase from Thermus aquaticus

A method is described for the purification of the enzyme aspartate aminotransferase from the thermophile Thermus aquaticus. The enzyme has been characterized with respect to its molecular weight on SDS PAGE and by amino acid analysis. Attempts to obtain N-terminal sequence data was unsuccessful, presumably due to a blocked N-terminus. The purified enzyme has been shown to be highly thermostable, having a half life of inactivation of about 6 hours at 100 degrees C, and to have a temperature optimum greater than 95 degrees C.

Functional integrity of the central and sympathetic nervous systems is a prerequisite for pressor and tachycardic effects of diphenyleneiodonium, a novel inhibitor of nitric oxide synthase

The pressor and tachycardic effects of diphenyleneiodonium (DPI), a novel inhibitor of endothelial nitric oxide synthase with chemical structure different from those of NG-substituted Arg analogs, were studied in pentobarbital-anesthetized rats. Bolus injections of DPI (0.05-1.6 mg/kg i.v.) caused transient (1-2 min in duration) and dose-dependent increases in mean arterial pressure (MAP) with ED50 of 0.22 +/- 0.02 mg/kg and maximum effect (Emax) of 58 +/- 3 mm Hg, and heart rate (HR) with ED50 of 0.26 +/- 0.03 mg/kg and Emax of 60 +/- 5 beats/min. Pretreatments with tetrodotoxin, reserpine, guanethidine, mecamylamine, but not atropine, rauwolscine, captopril nor L-Arg, attenuated the MAP and HR responses to DPI. Phentolamine and prazosin attenuated the MAP but not HR response whereas propranolol attenuated the HR but not MAP response of DPI. Pithing abolished, whereas spinal cord transection reduced, the MAP and HR responses to DPI. Pithing did not alter the pressor response but blocked the reflex bradycardic response to NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. Bolus injection of a single dose of DPI (1.6 mg/kg i.v.) or NG-nitro-L-arginine increased MAP, but only DPI caused immediate and large increases (> 1 ng/ml) in plasma norepinephrine, epinephrine and moderate increase in dopamine; pretreatment with reserpine attenuated, whereas pithing abolished these increases. The increases in plasma norepinephrine and epinephrine by DPI were positively correlated to increases in MAP and HR. The results demonstrate that DPI, unlike NG-substituted Arg analogs, produces pressor and tachycardic effects via indirect activation of the sympathetic nervous system.

In vitro and ex vivo inhibitory effects of L- and D-enantiomers of NG-nitro-arginine on endothelium-dependent relaxation of rat aorta

The in vitro and ex vivo inhibitory effects of NG-nitro-L-arginine (L-NNA) and NG-nitro-D-arginine (D-NNA) on endothelium-dependent relaxations were studied in rat aortic rings. L-NNA (3 x 10(-7) to 3 x 10(-5) M) but not D-NNA (3 x 10(-6) to 3 x 10(-4) M) induced contraction of resting aortic rings and potentiated phenylephrine-induced contraction in a concentration-dependent manner. In phenylephrine-preconstricted aortic rings, L-NNA (3 x 10(-7) to 3 x 10(-5) M) and D-NNA (3 x 10(-6) to 3 x 10(-4) M) concentration-dependently inhibited the relaxation response to acetylcholine (ACh) with similar efficacies and IC50 values of 10(-6) and 3.9 x 10(-5) M, respectively. In addition, both L-NNA (3 x 10(-5) M) and D-NNA (3 x 10(-4) M) almost totally inhibited the relaxation of preconstricted rings by the calcium ionophore A 23187. The inhibitory effects of L- and D-NNA remained for at least 4 hr after the preparations were washed out. Neither the inhibitory effects of L- and D-NNA on ACh-induced relaxation nor the ACh-induced relaxation itself were affected by pretreatment with indomethacin. However, pretreatment (10 min) or post-treatment (1 hr later) with L-Arg (10(-3) M) completely prevented or markedly reversed the inhibitory effects of L- and D-NNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression by ethanol of pressor response caused by the inhibition of nitric oxide synthesis

The effects of ethanol on mean arterial pressure (MAP) and heart rate (HR) responses to the nitric oxide synthase inhibitor, NG-nitro-L-arginine, and to angiotensin II and noradrenaline were studied in rats. I.V. bolus injections of NG-nitro-L-arginine dose dependently increased MAP in vehicle-pretreated rats, with a maximum increase of 56 +/- 7 mm Hg and an ED50 of 3.8 +/- 0.4 mg/kg, respectively. I.v. infusions of ethanol dose dependently reduced maximum MAP response to NG-nitro-L-arginine, with a Ki of 96 +/- 8 mg/kg per min but did not alter the ED50. Ethanol (48 mg/kg per min) did not modify the MAP response to i.v. bolus injections of angiotensin II (0.02-1.28 micrograms/kg) or noradrenaline (0.25-16 micrograms/kg). However, ethanol attenuated the reflex HR responses of NG-nitro-L-arginine and angiotensin II but not that of noradrenaline. The results demonstrate that ethanol selectively but non-competitively inhibits the MAP response to NG-nitro-L-arginine, suggesting an interaction between ethanol and the L-arginine/nitric oxide pathway.

Developmental expression of the platelet-derived growth factor alpha-receptor gene in mammalian central nervous system

We recently reported that the platelet-derived growth factor (PDGF) A-chain gene is highly expressed in neurons of embryonic and adult mouse central nervous system and suggested that its secretion by neurons may support development and maintenance of glia. We have now analyzed the levels and sites of expression of the cognate PDGF alpha-receptor gene in brain and spinal cord of embryonic and adult mice by in situ hybridization. The predominant cell populations in both gray and white matter expressing transcripts of the PDGF alpha-receptor gene are glial cells or their precursors. Transcripts consistently were not detected in neurons. Expression of the PDGF alpha-receptor gene was first observed at embryonic day 15, increased through postnatal day 14, and fell to lower levels in adults. Expression of the alpha-receptor gene corresponds in temporal sequence to the developmental period of glial migration and proliferation and to the expression of PDGF A by neurons. The results indicate that glia but not neurons have the potential to respond to PDGF A and suggest that neurons influence glial cell development through paracrine regulation.

A comparison of the inhibitory effects of sodium nitroprusside, pinacidil and nifedipine on pressor response to NG-nitro-L-arginine

1. The inhibitory effects of sodium nitroprusside (SNP), a nitric oxide (NO) donor, on mean arterial pressure (MAP) responses to NG-nitro-L-arginine (L-NNA) (NO synthase inhibitor), angiotensin II (AII) and noradrenaline (NA) were compared with those of pinacidil (KATP channel opener) and nifedipine (L-type calcium antagonist) in conscious, unrestrained rats. 2. Intravenous bolus injections of L-NNA (1-64 mg kg-1), AII (0.02-1.28 micrograms kg-1) and NA (0.25-16 micrograms kg-1) dose-dependently increased MAP to similar maxima. Intravenous infusions of SNP (1, 4 and 16 micrograms kg-1 min-1) dose-dependently increased ED20S of L-NNA, AII and NA. However, the maximum response evoked by L-NNA, but not by AII nor NA, was dose-dependently reduced by SNP. Moreover, the inhibitory effect of SNP on the pressor response to L-NNA ceased when the infusion of SNP was terminated. 3. Pinacidil (80 micrograms kg-1 min-1 for 30 min followed by 5 micrograms kg-1 min-1) increased ED50S of L-NNA, AII and NA but did not decrease the maximum responses to any of these agents. 4. Nifedipine (1 mg kg-1 min-1) non-selectively reduced maximum responses to L-NNA, AII and NA to similar levels and increased ED50S of AII and NA but not L-NNA. 5. The results show that SNP causes a selective, non-competitive and reversible inhibition of the pressor response to L-NNA. This inhibition by SNP is unlikely to be related to hypotension, the opening of ATP-sensitive potassium channels or blockade of L-type calcium channels.

The role of adenosine in glycine-induced glomerular hyperfiltration in rats

Ingestion of a high-protein diet or infusion of amino acids induces glomerular hyperfiltration and hyperemia. We have investigated the role of endogenous adenosine in glycine-induced hyperfiltration. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious chronically instrumented rats. Glycine (3.7 mg/min, i.v.; n = 6) significantly increased GFR and ERPF from 0.92 +/- 0.07 to 1.13 +/- 0.08 and 3.28 +/- 0.24 to 3.69 +/- 0.19 ml/min.100 g, respectively. In the presence of adenosine deaminase (ADA, 2 U/kg.min, n = 6), glycine-induced glomerular hyperfiltration and hyperemia were blunted. The small changes in GFR (from 0.86 +/- 0.06 to 0.90 +/- 0.10 ml/min.100 g) and ERPF (from 3.60 +/- 0.57 to 3.83 +/- 0.53 ml/min x 100 g) were not statistically significant. Erythro-9-(2-hydroxy-3-nonyl) adenosine hydrochloride (100 micrograms/kg.min, n = 6), an ADA inhibitor, reversed the effect of ADA. Injection of 8-phenyltheophylline (10 mg/kg, n = 6), an adenosine A1 receptor antagonist that alone did not affect GFR, abolished the glycine-induced glomerular hyperfiltration (GFR from 1.02 +/- 0.08 to 0.93 +/- 0.08 ml/min.100 g, P > .05). 8-phenyltheophylline, which itself decreased ERPF, also significantly decreased the ERPF response to glycine (3.47 +/- 0.26 to 2.78 +/- 0.14 ml/min x 100 g). Thus, endogenous adenosine, acting at adenosine A1 receptors, plays an important role in the glomerular hyperfiltration and hyperemia induced by glycine.

Characteristics of depressing effect of cycloprotobuxine-A on the maximal velocity of depolarization in myocardium

The characteristics of the depressing effect of cycloprotobuxine-A on Vmax in isolated guinea pig myocardium were investigated with a standard microelectrode technique. Cycloprotobuxine-A 3-30 mumol/l produced a concentration-dependent decrease in Vmax, associated with a prolongation of the action potential duration. At 3 mumol/l, a small resting block of Vmax, but a pronounced use-dependent block was found. This use-dependent block increased progressively as stimulation frequency increased from 0.5 to 4.0 Hz. The time constant and rate for onset of use-dependent block were 2.0-7.3 s and 0.11-0.25 AP-1 (reciprocal action potential number), respectively. The time constant and half-life for recovery from use-dependent block were 11.8 and 8.1 s. Thus, the kinetics of the depressing effect of cycloprotobuxine-A on Vmax were intermediate. In muscles depolarized by high [K+]0, the resting block was enhanced slightly, while use-dependent block was greatly augmented. These results suggested that cycloprotobuxine-A could have a low affinity for resting Na+ channels but a much higher affinity for activated and/or inactivated Na+ channels, and that the use-, frequency- and voltage-dependent effects of cycloprotobuxine-A on Vmax may play an important role in preventing the development of cardiac arrhythmias.

Dopamine DA1 receptor agonist, fenoldopam, reverses glycine-induced hyperfiltration in rats

Stimulation of dopamine DA1 receptors can prevent glomerular hyperfiltration in streptozotocin-induced diabetic rats. In the present study we have therefore investigated whether the DA1 agonist, fenoldopam, can prevent glycine-induced hyperfiltration. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by inulin and p-aminohippurate clearances in conscious chronically instrumented rats. Glycine (3.7 mg/min iv; n = 8) significantly increased GFR by 37% (from 1.09 +/- 0.53 to 1.49 +/- 0.11 ml.100 g-1.min-1, P less than 0.01), ERPF by 23% (from 2.96 +/- 0.30 to 3.64 +/- 0.43 ml.100 g-1.min-1, P less than 0.05), and filtration fraction (FF) by 13% (from 0.39 +/- 0.04 to 0.44 +/- 0.05, P less than 0.05). Fenoldopam, at a dose (1 microgram.kg-1.min-1 iv; n = 8) that increased ERPF by 26%, decreased FF by 13%, but did not change GFR, significantly attenuated the glycine-induced hyperfiltration. In the presence of fenoldopam, glycine resulted in only an 8% increase in GFR (from 1.08 +/- 0.07 to 1.17 +/- 0.09 ml.100 g-1.min-1; n = 8). ERPF increased by 20% (from 3.34 +/- 0.24 to 4.00 +/- 0.21 ml.100 g-1.min-1, P less than 0.05), and FF decreased by 13% (from 0.34 +/- 0.03 to 0.29 +/- 0.02, P less than 0.05). Infusion of the DA1-selective antagonist, Sch 23390, abolished the effects of fenoldopam. Thus DA1 receptor activation can prevent glomerular hyperfiltration induced by glycine.

Effects of anaesthetic agents on pressor response to beta-blockers in the rat

It has been shown that paradoxical pressor response to a beta-adrenoceptor antagonist occurs in conscious rats pretreated with an alpha-adrenoceptor antagonist. This study examines the influence of anaesthetic agents on mean arterial pressure (MAP) response to a beta-blocker. Cumulative dose-response curves of propranolol (non-selective), ICI 118,551 (beta 2-selective) and atenolol (beta 1-selective) were constructed in phentolamine-treated rats anaesthetized with urethane, pentobarbitone or halothane. I.v. injections of all three beta-blockers caused dose-dependent increases in MAP in urethane-anaesthetized rats. In halothane-anaesthetized rats, propranolol and atenolol did not alter MAP while ICI 118,551 caused a small dose-dependent increase in MAP. In the presence of pentobarbitone, none of the beta-blockers raised MAP. In the second series of experiments, a single i.v. bolus dose of propranolol was given in phentolamine-treated rats anaesthetized with pentobarbitone, amobarbitone, ketamine or chloralose. Propranolol did not affect MAP in rats anaesthetized with pentobarbitone, amobarbitone and chloralose but it partially reversed the hypotensive effect of phentolamine in ketamine-anaesthetized rats. In the third series, propranolol or atenolol was i.v. injected in pentobarbitone-anaesthetized rats treated with both phentolamine and adrenaline. Both propranolol and atenolol raised MAP. Our results show that anaesthetic agents differentially affect the MAP response to a beta-blocker.

Mechanism of the vasodilator action of calcitonin gene-related peptide in conscious rats

1. The aim of this study was to investigate whether the hypotensive effect of rat alpha-calcitonin gene-related peptide (alpha CGRP) in conscious rats is mediated by endothelium-derived nitric oxide (NO) or the opening of adenosine 5'-triphosphate (ATP)-sensitive potassium (KATP) channels. 2. Dose-mean arterial pressure (MAP)-response curves of alpha CGRP were examined in the presence of vehicle, phenylephrine, KATP channel antagonist glibenclamide or NO synthase inhibitors, NG-nitro-L-arginine methyl ester (L-NAME) and NG-nitro-D-arginine methyl ester (D-NAME). Dose-MAP-response curves for sodium nitroprusside were also constructed in the presence and absence of L-NAME and D-NAME. 3. alpha CGRP and nitroprusside produced dose-dependent reductions in MAP which were potentiated by phenylephrine. Both L-NAME and D-NAME attenuated the depressor response to alpha CGRP but not nitroprusside. 4. Dose-MAP-response curves for pinacidil, a KATP-channel activator, were also examined in the presence of glibenclamide or vehicle. Glibenclamide attenuated pinacidil- but not alpha CGRP-induced reductions in MAP. 5. It is concluded that the hypotensive effects of alpha CGRP are partially mediated via endothelium-derived NO but not via the opening of KATP channels.

Protective effect of cycloprotobuxine-A against cardiac arrhythmias induced by ouabain

Cycloprotobuxine-A (CPB-A) 1-4 mg.kg-1 iv increased the dose of ouabain required to induce ventricular arrhythmias in guinea pigs. At the equitoxic doses (1/50 LD50), CPB-A was more potent than cyclovirobuxine-D and amiodarone. Pretreatment with reserpine (5 mg.kg-1 ip), vagotomy or pithing spinal cord did not prevent the action of CPB-A, which indicate that the protective effect of CPB-A may be due to its direct action on myocardium without the involvement of nervous system. In isolated guinea pig ventricular muscles, CPB-A 3 mumol.L-1 consistently decreased the amplitude of oscillatory afterpotentials (OAP) and blocked triggered activity elicited by ouabain. At 30 mumol.L-1, CPB-A abolished the appearance of OAP. It seems that one of the mechanisms for the anti-arrhythmic action of CPB-A was a decrease in the amplitude of OAP.

Renin-angiotensin system inhibition reduces glycine-induced glomerular hyperfiltration in conscious rats

It has been reported that high protein intake or amino acid infusion-induced glomerular hyperfiltration are accompanied by an elevation of plasma renin activity and renal renin mRNA. We therefore investigated the effect of inhibition of the renin-angiotensin system by SK&F 108566, a novel, nonpeptide angiotensin II (AII) receptor antagonist, or by enalapril, an angiotensin converting enzyme inhibitor, on glycine-induced hyperfiltration. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by inulin and p-aminohippurate clearances in conscious chronically instrumented rats. Glycine infusion (3.7 mg/min i.v.; n = 8) significantly increased GFR by 27% (from 1.09 +/- 0.53 to 1.38 +/- 0.08 ml/min.100 g), ERPF by 22% (2.96 +/- 0.30 to 3.61 +/- 0.32 ml/min.100 g) and significantly decreased effective renal vascular resistance by 22% [from 25.4 +/- 2.9 to 20.8 +/- 2.5 mm Hg/(ml/min.100 g)]. SK&F 108566 (30 micrograms/kg.min) or enalapril (1 mg/kg), at doses which inhibited the pressor effects of AII or AI, respectively, but had no significant influence on base-line GFR and ERPF, significantly attenuated the glycine-induced glomerular hyperfiltration and hyperemia. In the presence of SK&F 108566 or enalapril, glycine resulted in only small, statistically insignificant changes in GFR (from 1.07 +/- 0.03 to 1.10 +/- 0.04 and from 1.19 +/- 0.03 to 1.21 +/- 0.08 ml/min.100 g, respectively), ERPF (from 3.27 +/- 0.21 to 3.53 +/- 0.26 and from 3.57 +/- 0.11 to 3.41 +/- 0.38 ml/min.100 g, respectively) and effective renal vascular resistance [from 21.2 +/- 1.9 to 19.2 +/- 1.6 and from 18.4 +/- 0.9 to 20.2 +/- 2.2 mm Hg/(ml/min.100 g], respectively).(ABSTRACT TRUNCATED AT 250 WORDS)