Taurine binding to the purified insulin receptor.

Taurine (2-aminoethanesulfonic acid) was shown to bind specifically and reversibly to the purified human insulin receptor. While insulin binding to the purified insulin receptor exhibited characteristic negative cooperativity and an apparent dissociation constant (Kd) of 1.2 X 10(-9) M, taurine binding was shown to exhibit positive cooperativity and had a lower affinity for the insulin receptor. The apparent Kd for taurine binding to the purified insulin receptor was calculated to be 130 X 10(-9) M and the maximum number of binding sites (Bmax) was 1.6 nmol/mg receptor protein. Chromatographic data demonstrated that taurine binds to the 138,000 molecular weight subunit of the insulin receptor. Taurine binding to the receptor protein was displaced by either taurine or insulin. Anti-human insulin receptor sera prevented insulin or taurine from binding to the receptor. Taurine binding to the protein was pH dependent, and sulfur-containing taurine analogues were able to displace taurine from the insulin receptor. These data supported our previous in vivo and in vitro observations that the hypoglycemic properties of taurine appear to be mediated through an interaction of taurine with the insulin receptor.

Naloxone reverses the serotonin dependent hypotensive action of CGP 6085 A.

CGP 6085 A, a specific 5-hydroxytryptamine (5-HT), serotonin uptake inhibitor, is also a potent hypotensive agent. Its depressor effect in the spontaneously hypertensive (SH) rats correlates well with its ability to inhibit serotonin uptake. Here we report that the effects of CGP 6085 A on arterial blood pressure were greatly reduced in rats pretreated with p-chlorophenylalanine (pCl-Phe), a specific depletor of serotonin. Moreover, in rats pretreated with the serotonin antagonist, methysergide, CGP 6085 A did not produce significant hypotension. We also found that centrally administered naloxone reverses the hypotensive effect of CGP 6085 A. These results provide further evidence for the existence of an important serotonin-opioid interaction in the mechanisms of arterial blood regulation in the rat.

Inhibition of rat brainstem monoamine oxidase activity by CGP 6085 A.

CGP 6085 A [4-(5,6-dimethyl-2-benzofuranyl)piperidine] HCl, a known serotonin inhibitor, also inhibits rat brainstem monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) in both in vivo and in vitro experiments. Serotonin (5-HT) deamination by MAO-A is inhibited 35% at a dose of 100 mg/kg i.p. in vivo. Similar experiments show a maximal 20% decrease in phenylethylamine (PEA) deamination by MAO-B at a dosage of 30 mg/kg i.p. Over the range of 0.1 to 10 mg/kg i.p., CGP 6085 A decreases 5-HIAA levels in the brainstem. This in vivo inhibition of MAO activity is confirmed by in vitro experiments. In vitro studies in rat brainstem mitochondrial preparations show a dose-dependent, reversible, inhibition of MAO using tyramine as the substrate for the enzyme reaction. With an in vitro IC50 of 2-3 microM, the potency of CGP 6085 A is comparable to pargyline.

Enhanced sympathetic-adrenal medullary response to cold exposure in spontaneously hypertensive rats.

To investigate the regulation of sympathetic-adrenal medullary function in spontaneously hypertensive (SHR) male rats, we measured urinary catecholamine excretion for 4 h at room temperature and also during cold exposure (4 degrees C) in groups of four and 12-week-old stroke-prone SHR (SHRSP), stroke-resistant SHR (SHRSR) and normotensive Wistar-Kyoto (WKY) rats. The effect of cold exposure on 12-week-old adrenal denervated rats was also examined. At room temperature, urinary excretion of epinephrine, but not norepinephrine or dopamine, was increased significantly in four-week-old SHRSP and SHRSR rats compared with age-matched WKY. The enhanced excretion of epinephrine at room temperature was not observed in hypertensive rats at 12 weeks of age. During cold exposure, urinary concentrations of each catecholamine increased markedly in rats of all three strains. In addition, the epinephrine response was significantly enhanced in SHRSP rats and the norepinephrine, epinephrine and dopamine responses were significantly enhanced in SHRSR rats. Following adrenal denervation, the urinary epinephrine response to cold exposure was abolished in all strains. These results reveal an enhancement of sympathetic and neurally-mediated adrenal medullary responses in prehypertensive SHR rats and a greater urinary epinephrine response to cold exposure in four and 12-week-old SHR rats. This alteration in catecholamine secretion may be important in the development and maintenance of this type of experimental hypertension.

The hypotensive action of 4-(5,6-dimethyl-2-benzofuranyl) piperidine HCl (CGP 6085 A) in spontaneously hypertensive rats.

CGP 6085 A, [4-(5,6-dimethyl-2-benzofuranyl)piperidine HCl], has been found to be a mild to moderately potent hypotensive agent. One hour following CGP 6085 A administration (10 mg/kg, i.p.), a maximal reduction in blood pressure of approximately 20-30 mm Hg is observed in spontaneously hypertensive rats. The maximal reduction in blood pressure was observed at a dose of 3 mg/kg. CGP 6085 A blocks 5-HT uptake in the brainstem when assessed in vivo by use of the serotonin depletor, H 75/12 (3-hydroxy-4-methyl-alpha-ethyl-phenylethylamine). The maximal inhibitory effect on 5-HT uptake occurred at 10 mg/kg CGP 6085 A. The reduction in blood pressure correlates well with the ability of the drug to inhibit 5-HT uptake as assayed by H 75/12, with a correlation coefficient of 0.71 for SH rats. However, since the drug has not been widely characterized, alternate explanations for the cardiovascular pharmacological properties of CGP 6085 A are also proposed.

Hypoglycemic properties of taurine: not mediated by enhanced insulin release.

Taurine (2-aminoethanesulfonic acid) has been shown to be a potent hypoglycemic agent in the Wistar Kyoto rat (WKY). Glucose and insulin levels were measured in serum at various times after glucose loading (400 mg/kg, i.p.). Pretreatment with taurine (200 mg/kg, i.p.) attenuated the rise in serum glucose levels at 0.5 hr after glucose administration. In addition, taurine also prevented the rise in serum immunoreactive insulin levels. The taurine analogue hypotaurine produced a similar inhibition in the rise of both serum glucose and insulin levels while beta-alanine, the carboxylic acid derivative of taurine, was totally ineffective. The enhanced glucose clearance can be explained by an increase in deoxyglucose accumulation in skeletal muscle and liver. In the liver, a 50% increase in glycogen synthesis was observed. A possible interrelationship between taurine and insulin receptor is discussed.

The hypothermic effect of 4-(5,6-dimethyl-2-benzofuranyl) piperidine HCl (CGP 6085 A) in Wistar Kyoto rats.

CGP 6085 A [4-(5,6-dimethyl-2-benzofuranyl) piperidine HCl], a reported serotonin uptake and MAO (16) inhibitor, is a potent hypothermic agent. The hypothermic action of CGP 6085 A is dose dependent with a maximal reduction in rectal core temperature of greater than 1 degree C within one hour after drug administration. Fluoxetine and citalopram elicit a similar response at equal doses. These results suggest that inhibition of serotonin uptake may produce the hypothermic effect. To assess the in vivo action of CGP 6085 A in inhibiting hypothalamic serotonin uptake, CGP 6085 A (10 mg/kg) was injected one hour prior to injection of 3-hydroxy-4-methyl-alpha-ethyl-phenylethylamine (H75/12), a serotonin depletor. The ability of CGP 6085 A to block the uptake of H75/12 by the 5HT uptake system was indicative of its ability to block serotonin uptake. Pretreatment with p-chlorophenylalanine (pCPA), an inhibitor of serotonin synthesis, resulted in the loss of the hypothermic response to CGP 6085 A. Thus, these data are consistent with the idea that CGP 6085 A may produce its hypothermic response by inhibiting serotonin uptake.

Action of substance P on the working rat heart.

The effect of substance P (SP) on the cardiodynamics of the isolated working rat heart perparation was examined. The peptide over the concentration range of 10(-8) to 10(-6) M was found to have no influence on aortic pressure, cardiac output, or cardiac work. However, a 10-15% reduction in coronary flow was observed at 1 x 10(-6) M substance P. Octapeptide substance P (SP4-11) exhibited a similar vasoconstrictive action. The IC50 of SP4-11 was 2 x 10(-13) M compared to an IC50 of 3.5 x 10(-8) M for substance P. Perfusion of the heart in the presence of bacitracin (1 x 10(-4) M), a protease inhibitor, prevented the reduction in coronary flow observed in the presence of substance P. By contrast, the reduction in coronary flow produced by octapeptide substance P was not altered by the presence of bacitracin. Thus, it appears that a C-terminal fragment such as SP4-11 may be responsible for the observed decrease in coronary flow.

Taurine enhancement of calcium binding to rat heart sarcolemma.

The effect of taurine on calcium binding to isolated rat heart sarcolemmal membrane was examined. Taurine was observed to increase calcium binding to the low affinity sites in both high sodium-low potassium and low sodium-high potassium buffers. Taurine was also seen to antagonize the inhibition of calcium binding to the sarcolemma caused by both verapamil and lanthanum. Nevertheless, membrane structural changes due to taurine could not be detected using the spin label ESR probe 2N14. A possible regulatory role of taurine is discussed.