Forskolin-stimulated Adenylate Cyclase Research Articles

Beta-receptors and adenylate cyclase: comparison of nonischemic, ischemic, and postmortem tissue

We compared the effects of myocardial ischemia and postmortem changes on beta-adrenergic receptors and their coupling to adenylate cyclase activity. The effects of 1 h of left circumflex coronary artery occlusion were examined in eight conscious calves, which were then anesthetized with pentobarbital sodium, and the left ventricle was divided into nonischemic and ischemic regions. A crude membrane fraction was prepared from each region and from the nonischemic tissue 1 h postmortem. beta-Adrenergic receptor density increased (152 +/- 55%) and decreases in basal (-21 +/- 6.1%), isoproterenol-stimulated (-25 +/- 8.0%), 5'-guanylylimidodiphosphate [Gpp(NH)p]-stimulated (-17 +/- 5.8%), fluoride-stimulated (-26 +/- 5.8%), and forskolin-stimulated (-31 +/- 8.4%) adenylate cyclase activities were observed in the ischemic myocardium compared with nonischemic myocardium. Similarly, in postmortem samples, beta-adrenergic receptor density rose 58 +/- 16%, whereas decreases in basal (-48 +/- 8.7%), isoproterenol-stimulated (-61 +/- 7.8%), Gpp(NH)p-stimulated (-58 +/- 7.0%), fluoride-stimulated (-64 +/- 6.1%), and forskolin-stimulated (-52 +/- 6.2%) adenylate cyclase activities were observed. Agonist-binding competition curves with isoproterenol were shifted, indicating that beta-adrenergic receptors were binding agonists with low affinity in both the ischemic and postmortem myocardium. The marked, but directionally opposite, changes in receptor density and adenylate cyclase that occur postmortem indicate the importance of prompt processing of tissues. The striking similarity in response of beta-adrenergic receptor agonist and antagonist binding and adenylate cyclase activity in ischemic and postmortem tissue raises the speculation that similar mechanisms may operate under both conditions.

Effects of chronic ethanol treatment and aging on brain phosphoinositide turnover and adenylate cyclase activity

Inositol phosphate accumulation and adenylate cyclase activity were investigated in the cortex of young and aged ethanol-treated rats. Three months of ethanol treatment of young rats decreased maximal stimulation of inositol phosphate accumulation by carbachol by 26%, from 494 +/- 76% of basal turnover in control animals to 396 +/- 54% in ethanol-treated animals (mean +/- SD). In aged rats ethanol-related changes were no longer observed but age-related changes were evident. EC(50) was significantly higher than in young animals and maximal stimulation was significantly lower. Basal adenylate cyclase activity in cortical membranes of all groups of animals was not different. Forskolin-stimulated adenylate cyclase activity was not affected by ethanol treatment, but was higher in aged animals. The activity of forskolin-stimulated adenylate cyclase in the presence of carbachol was higher in both young and aged ethanol-treated animals, when compared to young controls. These results suggest that both ethanol and aging impair the efficiency of receptor/effector coupling.

Effect of monovalent ions upon G proteins coupling muscarinic receptors to phosphoinositide hydrolysis in the rat cerebral cortex

It has been suggested that K +, Li + and F1 − affect the function of G proteins coupled to signal transducing enzymes. Lithium, at concentrations which were found to reduce forskolin-stimulated adenylate cyclase activity, was without effect on either membrane [ 3H]phosphatidylinositol-4,5-bisphosphate ([ 3H]PIP 2) hydrolysis measured in the absence or presence of 5′-guanylyl-imidodiphosphate (Gpp(NH)p), or (at ⩾ 2.3 mM Li +) upon the stimulation of rat cerebral cortical inositol phospholipid breakdown by either carbachol, noradrenaline or NaF measured at either 6 or 18 mM K +. The increase in assay [K +] greatly enhanced the inositol phospholipid response to carbachol but not to NaF. The inhibitory effect of carbachol upon forskolin-stimulated adenylate cyclase was not affected by raising the [K +] from 6 to 18 mM. At 6 mM K + (both in the absence and presence of 15 μM AlCl 3), the effects of carbachol and NaF upon inositol phospholipid breakdown were essentially additive, whereas at 18 mM K +, the breakdown response to carbachol (antagonised by pirenzepine with a pA 2 value of 7.6) was similar in the absence and presence of NaF. It is concluded that in the rat cerebral cortex: (a) Li + does not affect the function of either the phosphoinositide-specific phospholipase C enzyme itself or the G p coupled to this enzyme; (b) the difference between the additivity between NaF and carbachol seen at different assay [K +] may reflect the K +-dependent changes in the tetrodotoxin-resistant and tetrodotoxin-sensitive pathways of carbachol stimulation of inositol phospholipid breakdown reported by Gurwitz and Sokolovsky (1987, Biochemistry 26, 633); and (c) the effect of K + on muscarinic receptor-coupled inositol phospholipid breakdown is not found for muscarinic receptors inhibitorily coupled to adenylate cyclase. Evidence is also presented to suggest that NaF affects the dephosphorylation of the formed [ 3H]inositol polyphosphates.

Influence of Age on the Rhythm in Basal and Forskolin-Stimulated Adenylate Cyclase Activity of the Rat Heart

In male rats of three different ages (Group I: 7 weeks, Group II: 4 months, Group III: greater than 10 months) basal and forskolin-stimulated adenylate cyclase (AC) activity were investigated in rat heart ventricles of animals sacrificed at 8 time points within 24 hours of a day. Maximum (Emax) and half-maximum (EC50) stimulation by a water-soluble forskolin (FOR; 0.1-100 mumol/l) were determined from dose-response curves. In young rats (Group I) significant rhythmicity was found in basal and FOR-stimulated AC activity, which was successively reduced or abolished with age by a reduction in amplitude. Mean basal AC activity increased about 2-fold from Group I to Group II/III. Mean maximum in FOR stimulation was about 12-fold in Group I and about 7-fold in Group II and III. EC50-values displayed neither a significant rhythmicity in either group nor an age-dependency. It is concluded that the stimulatory potency of the catalytic subunit of the AC is reduced with age.

6-Substituted tricyclic partial ergoline compounds are selective and potent 5-hydroxytryptamine 1A receptor agents

A series of 6 tricyclic partial ergoline derivatives was analyzed using radioligand binding assays. Four agents (LY 178210, LY 254089, LY 197205, and LY 197206) display high affinity (K i 1.3 nM) for 5-hydroxytryptamine 1A (5-HT 1A) receptor binding sites labeled by [ 3H] 8-hydroxy-2-(d1-n-propylamino) t tetralin (8-OH-DPAT) and display ⩾ 150 fold selectivity for the 5-HT 1A over the 5-HT 1D receptor binding site. The most potent agent investigated, LY 178210, is essentially inactive (K i > 1500 nM) at a total of 12 other neurotransmitter receptor binding sites in the brain. Using a forskolin-stimulated adenylate cyclase assay as a model of 5-HT 1A receptor function, LY 178210 was found to display partial agonist activity which was blocked by 10 −5 M (-) pindolol. These data indicate that LY 178210 is a potent and selective 5-HT 1A receptor partial agonist.

Identification and characterization of somatostatin receptors in neonatal rat long bones

Somatostatin (somatotropin release inhibiting factor; SRIF) has widespread functions as a modulator of neural activity as well as of endocrine and exocrine secretion. In the present paper, the binding characteristics of somatostatin receptors have been investigated in rat long bones using the stable analogue, 125I-SDZ 204-090, as a ligand. Binding studies revealed the presence of a single class of high-affinity binding sites for 125I-SDZ 204-090 on cells prepared from neonatal rat long bones with an equilibrium dissociation constant (KD) of 70.1 +/- 8.2 pM (n = 3). An excellent correlation was found between the ability of various somatostatin analogues to inhibit growth hormone in pituitary cells and to displace the binding of 125I-SDZ 204-090 to the bone cell preparation, indicating that the receptors are very similar, if not identical. The localization of the somatostatin-binding sites was examined by autoradiography after labelling in vitro and in vivo. The binding sites were shown by both procedures to be selectively localized to the metaphysis of rat long bones. The labelling experiments in vivo indicate that these receptors can be reached in the living animal by circulating somatostatin analogues. In addition, the analogue SMS 201-995 inhibited the forskolin-stimulated adenylate cyclase activity in bone cell suspensions. These results suggest that somatostatin could be an important regulatory factor in bone metabolism.

Histidine regulation of cyclic AMP metabolism in cultured renal epithelial LLC-PK1 cells.

L-Histidine and imidazole (the histidine side chain) significantly increase cAMP accumulation in intact LLC-PK1 cells. This effect is completely inhibited by isobutylmethylxanthine (IBMX). Histidine and imidazole stimulate cAMP phosphodiesterase activity in soluble and membrane fractions of LLC-PK1 cells suggesting that the IBMX-sensitive effect of these agents to stimulate cAMP formation is not due to inhibition of cAMP phosphodiesterase. Histidine and imidazole but not alanine (the histidine core structure) increase basal, GTP-, forskolin-, and AVP-stimulated adenylate cyclase activity in LLC-PK1 membranes. Two other amino acids with charged side chains (aspartic and glutamic acids) increase AVP-stimulated but neither basal- nor forskolin-stimulated adenylate cyclase activity. This suggests that multiple amino acids with charged side chains can regulate selected aspects of adenylate cyclase activity. To better define the mechanism of histidine regulation of adenylate cyclase, membranes were detergent-solubilized which prevents histidine and imidazole potentiation of forskolin-stimulated adenylate cyclase activity and suggests that an intact plasma membrane environment is required for potentiation. Neither pertussis toxin nor indomethacin pretreatment alter imidazole potentiation of adenylate cyclase. IBMX pretreatment of LLC-PK1 membranes also prevents imidazole to potentiate adenylate cyclase activity. Since IBMX inhibits adenylate cyclase coupled adenosine receptors, LLC-PK1 cells were incubated in vitro with 5'-N-ethylcarboxyamideadenosine (NECA) which produced a homologous pattern of desensitization of NECA to stimulate adenylate cyclase activity. Despite homologous desensitization, histidine and imidazole potentiation of adenylate cyclase was unaltered. These data suggest that histidine, acting via an imidazole ring, potentiates adenylate cyclase activity and thereby increases cAMP formation in cultured LLC-PK1 epithelial cells. This potentiation requires an intact plasma membrane environment, occurs independent of a pertussis toxin-sensitive substrate and of products of cyclooxygenase, and is inhibited by IBMX. This IBMX-sensitive pathway does not involve either inhibition of cAMP phosphodiesterase activity or a stimulatory adenosine receptor coupled to adenylate cyclase.

Effect of Phosphate Deprivation on Enzymes of the Cyclic Adenosine Monophosphate Metabolism in Rat Proximal Convoluted and Proximal Straight Tubules

Phosphate deprivation causes a resistance to the phosphaturic effect of parathyroid hormone (PTH). The present study determined whether acute phosphate deprivation alters basal or stimulated activities of key enzymes of the cyclic adenosine monophosphate (cAMP) metabolism in microdissected proximal convoluted and proximal straight tubules, since blunted cAMP levels in these proximal subsegments might account for refractoriness to the effect of PTH on phosphate reabsorption in the proximal convoluted and proximal straight tubule segments. In the proximal convoluted tubules of rats fed a normal-phosphate diet (NPD), PTH increased the adenylate cyclase activity by tenfold. In the proximal convoluted tubule of rats fed a low-phosphate diet (LPD), PTH also increased the adenylate cyclase activity by tenfold. In addition, forskolin increased the adenylate cyclase activity to levels similar to PTH in the proximal convoluted tubule of rats fed NPD or LPD. In the proximal straight tubule of rats fed NPD, PTH resulted in an approximately fivefold increase in adenylate cyclase activity. In the proximal straight tubule of rats fed LPD, PTH resulted in a fourfold increase in adenylate cyclase activity. The forskolin-stimulated adenylate cyclase activity was markedly decreased (46%) in the proximal straight tubule of phosphate-deprived rats. The cAMP-phosphodiesterase activity in the proximal convoluted tubule was significantly increased by 26% in phosphate-deprived rats. The cAMP-phosphodiesterase activities in the proximal straight tubules from rats fed NPD or LPD were similar. We conclude that distinct differences in key enzymes of cAMP metabolism exist in the proximal convoluted and proximal straight tubule subsegments. Further, phosphate deprivation affects the cAMP-phosphodiesterase and adenylate cyclase activities differently in these nephron subsegments.(ABSTRACT TRUNCATED AT 250 WORDS)

MDL 73005EF: partial agonist at the 5-HT 1A receptor negatively linked to adenylate cyclase

MDL 73005EF has been recently described as a potent, highly selective 5-HT 1A ligand. Although proposed to act predominantly as an antagonist (M. Hibert, A.K. Mir, G. Maghioros, P. Moser, D.N. Middlemiss, M.D. Trickleband and J.R. Fozard, 1988, The pharmacological properties of MDL 73005EF: a potent and selective ligand at 5-HT 1A receptors, Br. J. Pharmacol. 93, 2P), we have demonstrated that MDL 73005EF also acts as a highly efficacious partial agonist at the 5-HT 1A receptor, based on its ability to inhibit forskolin-stimulated adenylate cyclase in rat hippocampal membranes. Compared with two structurally related 5-HT 1A partial agonists, the rank order of potency of MDL 73005EF in the FSC assay was comparable to affinity calculated by radioligand binding.

The local anaesthetic benzyl alcohol attenuates the α2-adrenoceptor-mediated inhibition of human platelet adenylate cyclase activity when stimulated by prostaglandin E1, but not that stimulated by forskolin

Treatment of human platelets with concentrations of benzyl alcohol up to 50 mM augmented adenylate cyclase activity when it was assayed in the basal state and when stimulated by prostaglandin E1 (PGE1), isoprenaline or NaF. Benzyl alcohol antagonized the stimulatory effect exerted on the catalytic unit of adenylate cyclase by the diterpene forskolin. Benzyl alcohol did not modify the magnitude of the inhibitory response when the catalytic unit of adenylate cyclase was inhibited by using either low concentrations of guanosine 5'-[beta gamma-imido]triphosphate, which acts selectively on the inhibitory guanine nucleotide-regulatory protein Gi, or during alpha 2-adrenoceptor occupancy, by using adrenaline (+ propranolol). Some 34% of the potent inhibitory action of adrenaline on PGE1-stimulated adenylate cyclase was obliterated in a dose-dependent fashion (concn. giving 50% inhibition = 12.5 mM) by benzyl alcohol, with the residual inhibitory action being apparently resistant to the action of benzyl alcohol at concentrations up to 50 mM. Treatment of membranes with benzyl alcohol did not lead to the release of either the alpha-subunit of Gi or G-protein subunits. The alpha 2-adrenoceptor-mediated inhibition of adenylate cyclase was abolished when assays were performed in the presence of Mn2+ rather than Mg2+ and, under such conditions, dose-effect curves for the action of benzyl alcohol on PGE1-stimulated adenylate cyclase activity were similar whether or not adrenaline (+propranolol) was present. We suggest that (i) alpha 2-adrenoceptor- and Gi-mediated inhibition of PGE1-stimulated adenylate cyclase may have two components, one of which is sensitive to inhibition by benzyl alcohol, and (ii) the Gi-mediated inhibition of forskolin-stimulated adenylate cyclase exhibits predominantly the benzyl alcohol-insensitive component.

Lack of Inotropic Effects of Neuropeptide Y in Human Myocardium

The sympathetic co-transmitter neuropeptide Y (NPY) inhibits cardiac contractility in vivo in the rat heart. We tested whether NPY alters the force of contraction or the activity of adenylate cyclase in the isolated human right atrium. Neuropeptide Y did not affect basal-or isoproterenol-stimulated force of contraction and did not inhibit forskolin-stimulated adenylate cyclase activity. We conclude that NPY does not directly decrease cardiac contractility in humans. The inhibitory cardiac effects of NPY are likely to be secondary to coronary vasoconstriction, presynaptic inhibition of catecholamine release, and/or baroreflex activity.

Chronic norepinephrine elicits desensitization by uncoupling the beta-receptor.

The goal of this study was to determine the mechanism of beta-adrenergic receptor desensitization after chronic elevation of circulating NE levels. Osmotic minipumps containing either NE or saline were implanted subcutaneously in dogs for 3-4 wk. Physiologic desensitization to isoproterenol was confirmed in conscious dogs, i.e., left ventricular dP/dt increased in response to isoproterenol (0.4 micrograms/kg per min) by 5,625 +/- 731 mmHg/s in control dogs with saline pumps, and significantly less, P less than 0.01, by 2,093 +/- 263 mmHg/s in dogs with NE pumps. Myocardial beta-adrenergic receptor density as determined with 125I-cyanopindolol binding was 49% higher (p less than 0.05) in the NE pump group. However, beta-adrenergic receptor agonist binding with isoproterenol demonstrated a significant shift into the low affinity state for the animals with NE pumps. Basal, GTP plus isoproterenol, 5'-guanylylimidodiphosphate, sodium fluoride, and forskolin-stimulated adenylate cyclase activity in the NE pump group were significantly depressed (P less than 0.05) by amounts ranging from 20 to 40%. The functional activity of the guanine nucleotide binding protein Gs was also reduced (P less than 0.05) in animals with NE pumps. Thus, the process of desensitization in response to chronic elevation of NE levels in intact, normal dogs does not involve a decrease in beta-adrenergic receptor density. Rather, it is characterized by reduced adenylate cyclase activation and uncoupling of the beta-adrenergic receptor in association with decreased activity of the GTP-coupling protein Gs.

Calcium and protein kinase C enhance parathyroid hormone- and forskolin-stimulated adenylate cyclase in ROS 17/2.8 cells

Both parathyroid hormone (PTH)- and forskolin-stimulated adenylate cyclase activities in ROS 17/2.8 cells are enhanced by increasing the medium concentrations of CaCl2 from 10(-5) M to 3 x 10(-3) M. The ED50 for CaCl2 for both PTH- and forskolin-stimulated activities are similar. The tumor-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA), a known activator of protein kinase C, also enhanced both PTH- and forskolin-stimulated adenylate cyclase. This action of PMA is specific for protein kinase C as phorbol esters that are not activators of protein kinase C had no effect on the system. The combined effects of PMA and CaCl2 were more than additive. The separate and combined effects of PMA and CaCl2 changed the rate of activation of the enzyme (Vmax) but did not modify the ED50 for PTH or for forskolin. PMA and CaCl2 both enhanced the potentiating effect of submaximal dose of forskolin on PTH-stimulated adenylate cyclase. It is concluded that calcium and PMA enhance PTH-sensitive adenylate cyclase and increase the production of cAMP by a mechanism that appears to involve the catalytic subunit of the enzyme and probably its interaction with a guanine nucleotide regulatory protein.

Species differences in the positive inotropic response to DPI 201-106, a novel cardiotonic agent

The positive inotropic activity of the novel cardiotonic DPI 201-106 was investigated in rat and guinea pig isolated hearts. For comparative purposes, the adenylate cyclase stimulant forskolin and the sodium channel agonist veratridine were also evaluated in both species. DPI 201-106 and veratridine produced greater inotropic effects in rat hearts than in guinea pig hearts, whereas forskolin produced comparable effects. In both species the inotropic response to DPI 201-106 and veratridine, but not forskolin, was reversed by the sodium channel antagonist tetrodotoxin. These results confirm that the positive inotropic effect of DPI 201-106 is due to stimulation of the sodium channel and demonstrate for the first time that species differences exist in the inotropic response to this novel cardiotonic drug.

Potentiation of the effects of dopamine in the rabbit isolated splenic artery by 3-isobutyl-l-methylxanthine or forskolin

Dopamine relaxes precontracted rabbit isolated splenic arteries via stimulation of dopamine DA1 receptors on the smooth muscle. However, in many preparations, the effect of dopamine is small. In an attempt to increase the effectiveness of dopamine and thus increase the proportion of usable preparations, we have examined the effects, on the sensitivity of dopamine, of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and the adenylate cyclase stimulant forskolin. In tissues pretreated with phenoxybenzamine, and in the presence of propranolol, IBMX (1 mumol/l) or forskolin (0.01 mumol/l) produced a small increase in the sensitivity to dopamine in otherwise insensitive preparations. The maximum relaxation of U-46619-induced tone produced by dopamine was increased to around 50%. However, after treatment with higher concentrations of IBMX (10 mumol/l) or forskolin (0.1 mumol/l), previously insensitive preparations relaxed readily to dopamine with EC50 values in the range 1-3 mumol/l. Similarly, in spontaneously dopamine-sensitive preparations IBMX (1 mumol/l) or forskolin (0.01 mumol/l) produced only a 2-3-fold parallel shift to the left in the dopamine concentration-effect curves. However, higher concentrations of IBMX (10 mumol/l) or forskolin (0.1 mumol/l) produced 10-20-fold parallel shifts to the left in dopamine concentration-effect curves. Higher concentrations of IBMX (100 mumol/l) or forskolin (1 mumol/l) abolished the ability of the spasmogen to contract the preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin affects the ability of G i to be ADP-ribosylated but does not elicit its phosphorylation in intact hepatocytes

Insulin inhibited the ability of activated pertussis toxin to catalase the ADP-ribosylation of alpha-G i in isolated plasma membranes in either the absence of added guanine nucleotides or in the presence of GTP. In contrast, when the non hydrolysable GTP analogue guanylyl-5′-imido-diphosphate (p[NH]ppG) was added to ribosylation mixtures, to inhibit the action of pertussis toxin in catalysing the ADP-ribosylation of alpha-G i, then the addition of insulin attenuated the action of p[NH]ppG causing an increase in alpha G i ribosylation. Pre treatment of intact hepatocytes with insulin had no effect on the subsequent ability of thiol-preactivated pertussis toxin to cause the ADP-ribosylation of alpha G i using isolated membranes from such cells. The ability of p[NH]ppG to inhibit forskolin-stimulated adenylate cyclase activity was attenuated in the presence of insulin. Insulin did not cause the phosphorylation of alpha G i in either intact hepatocytes or in isolated membranes.

Inhibition of adenylate cyclase in rat brain synaptosomal membranes by GTP and phenylisopropyladenosine is enhanced in hypothyroidism

1. Synaptosomal membranes were isolated from rats made hypothyroid by treatment with propylthiouracil and a low iodine diet. 2. When assayed in the presence of 100 mM-Na+, inhibition of forskolin-stimulated adenylate cyclase by GTP was enhanced in membranes from hypothyroid animals. 3. Hypothyroidism also enhanced inhibition of adenylate cyclase by phenylisopropyladenosine (with 100 mM-Na+ and 10 microM-GTP present). 4. Hypothyroidism did not increase binding of the A1 adenosine receptor agonist phenylisopropyladenosine to synaptosomal membranes; rather, the maximum binding was slightly decreased without any change in the KD. 5. The effect of GTP in modifying the displacement of the antagonist [3H]diethylphenylxanthine from synaptosomal membranes by unlabelled phenylisopropyladenosine was more pronounced in the hypothyroid state. 6. These findings are consistent with hypothyroidism causing modification of the brain adenylate cyclase system at the level of the coupling protein Gi.

Regulation by High Density Lipoproteins of Muscarinic Acetylcholine Receptor Function in Chick Heart Cells Cultured in Defined Medium

Activation of cardiac muscarinic acetylcholine receptors (mAChR) on cultured chick heart cells results in a decrease in cellular cAMP levels and a stimulation of phosphoinositide breakdown. A serum-free culture system has been used to investigate the regulation of mAChR number and function by purified serum high density lipoprotein (HDL). Administration of HDL purified from rooster serum to chick heart cells cultured in defined medium results in an attenuation of the ability of muscarinic agonist to inhibit forskolin-stimulated cAMP accumulation, with no change in its ability to stimulate phosphoinositide hydrolysis or to mediate down-regulation of receptor number. The inclusion of HDL in the culture medium did not result in appreciable changes in mAChR number or affinity, nor were the levels of the inhibitory guanine nucleotide-binding regulatory proteins (G-proteins) altered. However, the ability of guanine nucleotides to inhibit forskolin-stimulated adenylate cyclase activity was reduced by HDL treatment, suggesting that HDL interferes with the capacity of G-proteins to interact with adenylate cyclase. In order to determine which component of native HDL mediates the decreased effectiveness of carbachol, the ability of lipid and apoprotein fractions to mimic the effect of HDL was tested. HDL lipid fractions were able to mimic the effect of native HDL, while protein fractions were not. This result suggests that the ability of HDL to attenuate muscarinic receptor function is mediated by its lipid constituents. The effect of HDL and HDL lipid fractions were not correlated with changes in membrane cholesterol content.

5-HT1D receptors in guinea-pig and pigeon brain

The characteristics of high affinity [3H]5-HT (5-hydroxytryptamine) binding to non 5-HT1A non 5-HT1C sites were examined in crude membranes prepared from different regions of guinea-pig and pigeon brains. The coupling of these sites to adenylate cyclase was examined, and its pharmacological profile investigated. In the presence of 100 nmol/l 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) and 100 nmol/l mesulergine, [3H]5-HT labelled with nanomolar affinity an apparently homogeneous population of recognition sites in guinea-pig and pigeon brain membranes. The rank order of affinities of agonists and antagonists (5-CT (5-carboxamidotryptamine) greater than 5-HT greater than RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydro-4- pyridinyl)-1H indole succinate) greater than yohimbine greater than or equal to rauwolscine greater than DP-5-CT (N,N dipropyl-5-carboxamidotryptamine) greater than or equal to mianserin greater than 8-OH-DPAT greater than mesulergine greater than SDZ 21-009 ((+/-)-4(3-tert-butyl-amino-2-hydroxypropoxy)-indol-2 carbonic acid isopropyl ester) greater than (-)propranolol), as well as their individual pKD values, were very similar to those at porcine caudate 5-HT1D sites and clearly different from those at rat cortex 5-HT1B sites. In the substantia nigra of the guinea-pig the 5-HT receptor-mediated inhibition of forskolin-stimulated adenylate cyclase had a pharmacological profile fully comparable to that of 5-HT1D binding sites (5-CT greater than 5-HT greater than yohimbine greater than RU 24969 greater than 8-OH-DPAT greater than SDZ 21-009 = isamoltane greater than (-)pindolol greater than (-)propranolol).(ABSTRACT TRUNCATED AT 250 WORDS)

The enhancement of carbachol‐induced salivary secretion by VIP and CGRP in rat parotid gland is mimicked by forskolin

Low doses of vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP) have been shown to augment the salivary volume secretion evoked by muscarinic receptor agonists or substance P (SP) in rat parotid gland. Since VIP and CGRP are known to activate adenylate cyclase, we have studied whether forskolin, which directly activates this enzyme, can mimic the effects of these peptides on salivary secretion from the parotid gland in anaesthetized (Inactin 0.5 g kg-1 i.p.) rats. We have also studied the effect of the secretagogues and peptides on glandular blood flow as revealed by the laser Doppler technique. Carbachol (5 nmol kg-1) and SP (185 pmol kg-1) injected i.v. caused a transient increase in parotid blood flow and a decrease in systematic blood pressure concomitant with a salivary secretion of 3.3 +/- 0.3 mg (n = 22) and 18.7 +/- 1.9 mg (n = 25) respectively. VIP (150 pmol kg-1) and CGRP (25 pmol kg-1) also caused a transient increase in glandular blood flow concomitant with a decrease in systemic blood pressure, but no salivary secretion. The glandular blood flow increased by 166 +/- 5% and 43 +/- 11% for VIP and CGRP respectively. When carbachol was given 20 s after the injection of VIP or CGRP, the secretory response was increased by about 250% and 60% respectively. The adenylate cyclase stimulator forskolin (2.5 pmol kg-1) produced the same type of response regarding blood flow and systemic blood pressure as VIP and CGRP and potentiated the salivary secretion evoked by carbachol (5 nmol kg-1) by about 100%.(ABSTRACT TRUNCATED AT 250 WORDS)