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Biomedical subjects

D P Westfall

Publications and source records attributed to D P Westfall.

At least 19 recordsLinked to original sources
PubMed1992

Involvement of cyclic GMP in non-adrenergic, non-cholinergic inhibitory neurotransmission in dog proximal colon.

1. Nitric oxide (NO) may serve as a non-adrenergic, non-cholinergic (NANC) neurotransmitter released from enteric inhibitory nerves in the gastrointestinal tract. We tested whether guanosine 3':5'-cyclic monophosphate (cyclic GMP) may serve as a second messenger in transducing the NO signal into inhibitory junction potentials (i.j.ps) and relaxation in the canine proximal colon. 2. The membrane permeable analogue of cyclic GMP, 8-bromo cyclic GMP (8-Br-cyclic GMP) mimicked the effects of NO by hyperpolarizing cells near the myenteric border of the circular muscle layer and shortening slow waves in cells near the submucosal surface of the circular muscle layer. 8-Br-cGMP also inhibited spontaneous phasic contractions. 3. The specific cyclic GMP phosphodiesterase inhibitor, M&B 22948, hyperpolarized cells near the myenteric border and prolonged the duration of i.j.ps. M&B 22948 also inhibited phasic contractile activity. 4. Methylene blue failed to reduce significantly the amplitude and duration of i.j.ps and had variable effects on contractions. 5. Cyclic GMP levels were assayed in unstimulated muscles and in muscles exposed to exogenous NO and electrical field stimulation. Both stimuli hyperpolarized membrane potential, inhibited contractions, and elevated cyclic GMP levels. 6. Treatment of muscles with L-NG-nitroarginine methyl ester (L-NAME) increased spontaneous contractile activity and lowered cyclic GMP levels. The inhibitory effect of M&B 22948 on contractions was greatly reduced after muscles were treated with L-NAME. 7. These data support the concept that the effects of NANC nerve stimulation and NO (which may be one of the enteric inhibitory transmitters) may be mediated by cyclic GMP.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1992

Nonadrenergic, noncholinergic inhibition and rebound excitation in canine colon depend on nitric oxide.

Nonadrenergic, noncholinergic (NANC) nerves regulate slow waves along the submucosal border of the canine proximal colon. Experiments were performed to determine the role of nitric oxide (NO) in NANC responses. NANC responses are characterized by hyperpolarization and reduction in slow-wave amplitude and duration during the period of stimulation. This is followed by a "rebound" excitation (increase in amplitude and duration) of the slow wave immediately after the stimulus. These responses were blocked by L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase and by tetrodotoxin (TTX). Exogenous NO mimicked NANC responses, including the poststimulus rebound excitation. Responses to NO were unaffected by L-NAME or TTX. Responses to NANC nerve stimulation and NO were blocked by oxyhemoglobin but not by methemoglobin. Rebound excitation was reduced by pretreatment with indomethacin, suggesting that an eicosanoid may mediate this phase of NANC responses. Taken together, these data suggest that NO mediates NANC nerve responses in the proximal colon. NO appears to directly cause the inhibitory response, but the rebound response may depend on release of an eicosanoid.

Animals↗
PubMed1991

Participation by purines in the modulation of norepinephrine release by methoxamine.

The alpha 1-receptor agonist methoxamine reduced, by a prazosin sensitive mechanism, the nerve stimulation evoked release of norepinephrine in the rat caudal artery. The effect of methoxamine was also antagonized by the purinoceptor antagonist 8-(p-sulfophenyl)theophylline suggesting an involvement of endogenous purines in this process. Indeed, methoxamine caused the release of adenine nucleotides and adenosine, an action which was blocked by prazosin. These results suggest that methoxamine releases ATP or a related purine which in turn decreases transmitter release by acting on prejunctional purinoceptors.

Adenine Nucleotides↗
PubMed1991

Norepinephrine increases Na-Ca exchange in rabbit abdominal aorta.

Na-Ca exchange was measured as intracellular Na+ (Na+i)-dependent 45Ca uptake in rabbit abdominal aortic rings. The amount of Na+i-dependent 45Ca uptake was proportional to both the concentration of Na+ in the Na(+)-loading solution and the concentration of Ca2+ in the assay medium. Na+i-dependent 45Ca uptake was inhibited by incorporation of Na+ in the assay medium and by amiloride analogues. Norepinephrine significantly enhanced the rate of 45Ca uptake in Na(+)-loaded tissue but had no effect on Na+i-independent 45Ca uptake. The effect of norepinephrine was prevented by phentolamine but not by propranolol. The stimulatory effect of norepinephrine was absent when the concentration of extracellular Ca2+(Ca2+o) was 0.3 mM or lower but became significant at 0.6 mM and higher. Na-Ca exchange was also increased by phorbol 12,13-dibutyrate but not by its inactive analogue (4 alpha-phorbol 12,13-didecanoate). 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of protein kinase C, blocked the stimulatory effect of norepinephrine on Na-Ca exchange. It is suggested that alpha-adrenoceptor stimulation increases Na-Ca exchange in rabbit abdominal aorta in a Na+i- and Ca2+o-dependent fashion. This effect is possibly mediated through the activation of protein kinase C.

Animals↗
PubMed1991

Smooth muscle adenosine A1 receptors couple to disparate effectors by distinct G proteins in pregnant myometrium.

We have previously shown that adenosine, acting at an A1 receptor, contracts the smooth muscle of virgin guinea pig uterus (M. A. Smith, I. L. O. Buxton, and D. P. Westfall. J. Pharmacol, Exp. Ther. 247: 1059-1063, 1988) and is not coupled to the expected inhibition of adenylate cyclase (M. A. Smith, J. L. Silverstein, D. P. Westfall, and I. L. O. Buxton. Cell. Signal. 1: 357-365, 1989). To probe the importance of contractile actions of adenosine in uterine smooth muscle and to further characterize the signal transduction pathway involved in A1-receptor action, we have studied the adenosine receptor and its coupling in pregnant guinea pig myometrium. Adenosine agonist and antagonist radioligands bind to saturable sites of the A1 subtype homogeneously distributed in the smooth muscle of pregnant guinea pig uterus. Agonist competition of antagonist radioligand binding in both the absence and presence of guanine nucleotide reveals high and low agonist affinity states of the receptor. Pretreatment of tissues with pertussis toxin (PTx) shifts the high-affinity sites to a lower affinity but does not affect low-affinity sites, whereas agonist competition in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is indistinguishable from the control, which is consistent with coupling of A1 receptors to both PTx-sensitive and PTx-insensitive GTP-binding proteins. Adenosine receptor inhibition of adenylate cyclase activity is prevented after pretreatment of the tissue with PTx, whereas increased inositol phosphate production is not. The data presented here are consistent with coupling of the A1 receptor to dual effectors in the pregnant state of the smooth muscle. The unique action of an A1 receptor to contract mammalian smooth muscle and the appearance, only in the pregnant state, of coupling to adenylate cyclase inhibition suggest a role for adenosine in parturition biology.

Adenosine↗
PubMed1991

Nitric oxide and nitrosocysteine mimic nonadrenergic, noncholinergic hyperpolarization in canine proximal colon.

Previous evidence suggests that nonadrenergic, noncholinergic (NANC) inhibitory neurotransmission in visceral muscles may be mediated by nitric oxide (NO). We have demonstrated that NO and the NO carrier S-nitrosocysteine can mimic the hyperpolarization in colonic muscle caused by nerve stimulation. The finding that S-nitrosocysteine breaks down fast enough to cause inhibitory junction potential (IJP)-like hyperpolarizations suggests that NO could be stored as a nitrosothiol in secretory vesicles in nerve terminals. Oxyhemoglobin blocked hyperpolarization responses to NO and S-nitrosocysteine and NANC IJPs. These findings suggest that NO is a biologically active transmitter substance in NANC inhibitory neurotransmission. NO enhanced the open probability of Ca(2+)-activated K+ channels in isolated colonic muscle cells. These channels may mediate the hyperpolarization response to NANC neurotransmission in colonic muscles.

Animals↗
PubMed1991

Nucleotide modulation of norepinephrine release from sympathetic nerves in the rat vas deferens.

The effects of a number of purinoceptor agonists and antagonists on norepinephrine (NE) overflow were examined in the electrically field-stimulated rat vas deferens. The P1 receptor agonists adenosine and 2-chloroadenosine and the P2 receptor agonists ATP and beta, gamma-methylene ATP all reduced the overflow of NE, which was quantified by high-performance liquid chromatography-electrochemical detection techniques. The P1 receptor antagonist 8-(p-sulfophenyl)-theophylline (8-SPT) and the P2 receptor desensitizing agent alpha, beta-methylene ATP blocked the inhibitory effects of both P1 and P2 receptor agonists. The pyrimidine nucleotide UTP also inhibited NE overflow and this effect was antagonized by 8-SPT. The adenosine uptake inhibitor S-p-nitrobenzyl-6-thioguanosine potentiated and adenosine deaminase blocked the inhibitory effect of adenosine on NE overflow but neither had any effect on the ability of the adenine nucleotides to inhibit NE overflow. These results indicate that adenine nucleotides can act per se, without conversion to adenosine, on a prejunctional receptor to inhibit the release of NE. Because the effects of the adenine nucleotides are antagonized by 8-SPT, it appears that they act at the same receptor as the adenine nucleosides. UTP apparently acts at this receptor as well. These findings suggest that prejunctional purinoceptors on the sympathetic nerves of the rat vas deferens differ from P1 or P2 receptors as usually defined and thus may represent a unique class of receptor (P3) as has been suggested for the prejunctional receptors of the rat caudal artery.

Adenosine↗
PubMed1990

Nerve and drug-induced release of adenine nucleosides and nucleotides from rabbit aorta.

The purpose of the present study was to assess the possible sites which contribute to the nerve stimulation- and alpha-agonist-induced overflow of endogenous adenine nucleosides and nucleotides in vascular tissue. Particular attention was focused on the endothelium because it is known that endothelial cells have a high concentration of ATP and its metabolites. Segments of rabbit thoracic aorta, some denuded of endothelial cells by rubbing the lumen of the vessel, were incubated in organ baths and subjected to transmural nerve stimulation or stimulated with the alpha-1 adrenoceptor agonist methoxamine. A portion of the bathing solution was processed for the determination of norepinephrine by high-performance liquid chromatography with electrochemical detection and a portion for determination of ATP, ADP, AMP and adenosine by high-performance liquid chromatography with fluorescence detection. Transmural stimulation led to a significant release of both norepinephrine and the adenine nucleosides and nucleotides in a ratio of 1 to 350. Removal of the endothelium did not change the release of norepinephrine but reduced the release of adenosine and its derivatives by 90%. Methoxamine also caused the release of adenosine and the adenine nucleotides which was reduced by 93% by removal of the endothelium. Thus, the endothelium seems to be a major source of transmural nerve stimulation and alpha-agonist induced overflow of adenosine and adenine nucleotides. The endothelium is not the exclusive source of these purine congeners, however. In the case of transmural stimulation there is approximately 10% of the total which is independent of the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine Nucleotides↗
PubMed1990

Effects of alpha,beta-methylene ATP on the prejunctional purinoceptors of the sympathetic nerves of the rat caudal artery.

The effects of alpha,beta-methylene ATP, an agent known to stimulate and then to desensitize P2-purinoceptors, on the release of endogenous norepinephrine from the electrically stimulated rat caudal artery were determined. Norepinephrine was quantified by high-performance liquid chromatography-electrochemical detection techniques. alpha,beta-Methylene ATP over the concentration range of 1 to 100 microM did not affect the release of norepinephrine evoked by stimulation for 3 min at 1 Hz. In contrast, 2-chloroadenosine, a P1 receptor agonist, and beta,gamma-methylene ATP, a P2 receptor agonist, produced a concentration-related inhibition of the release of norepinephrine presumably by activating prejunctional purinoceptors. The failure of alpha,beta-methylene ATP to inhibit transmitter release was apparently not related to the length of pretreatment with this agent because pretreatments of 0.5 to 15 min yielded similar results. These findings indicate that the ability of alpha,beta-methylene ATP to decrease excitatory junction potentials and vasoconstriction of the caudal artery, as reported by others, is not due to a decrease in the release of transmitter. In spite of not possessing agonistic properties, alpha,beta-methylene ATP does interact with prejunctional purinoceptors as judged by the finding that the inhibitory effects of 2-chloroadenosine and beta,gamma-methylene ATP, but not those of the alpha-2 agonist clonidine, were antagonized by alpha,beta-methylene ATP. alpha,beta-Methylene ATP thus appears to be an antagonist at prejunctional purinoceptors (classified by us previously as P3-purinoceptors). alpha,beta-Methylene ATP also appears to act as an antagonist against endogenously released adenine nucleosides and nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Chloroadenosine↗
PubMed1989

Dissociation between adenosine receptors and adenylate cyclase in the smooth muscle of guinea pig myometrium.

We have previously demonstrated that adenosine causes contraction of guinea-pig myometrium in a fashion consistent with the presence of a purinergic receptor of the A1 subtype. Incubation of guinea-pig uterine smooth muscle membranes with the stable adenosine analogue [3H]cyclohexyladenosine [( 3H]CHA) resulted in rapid, reversible association of radioligand to saturable sites. The affinity (KD) of the receptor for [3H]CHA determined from kinetic experiments (3.14 nM) is in good agreement with that determined in saturation experiments (KD = 4.5 nM). Scatchard analysis of specific [3H]CHA binding (Bmax = 79 fmol/mg protein) is consistent with a single class of binding sites for [3H]CHA. Computer analysis of competition of [3H]CHA binding by the stereoisomers of phenylisopropyl adenosine, R-PIA (KI = 5.3 nM) and S-PIA (KI = 69 nM), as well as the 5'-substituted analogue, ethylcarboxamide adenosine (NECA; KI = 4.2 nM) suggest that [3H]CHA binding occurs to a single class of receptors of the AI subtype. Contractile studies employing these agents reveal that the relative order of potency, based on ED50 values, correlates well with the relative order of competition of agonist binding, based on equilibrium binding constants. Direct assay of myometrial adenylate cyclase failed to show that adenosine receptors in this smooth muscle are coupled to adenylate cyclase. We conclude here that a smooth muscle adenosine receptor is not coupled to adenylate cyclase, yet subserves muscle contraction. These data are important in light of recent attempts to classify adenosine receptors as dual regulators of adenylate cyclase.

Adenosine↗
PubMed1989

Factors involved in the generation of tension during contraction to high potassium in the rat vas deferens.

A large number of studies indicate that K(+)-induced contractions of smooth muscle depend on extracellular calcium. If these contractions depend exclusively on extracellular calcium then contractile responses to 140 mM K+, which are larger than the response to 35 mM K+, should be associated with a larger influx of 45Ca. This is not the case in the vas deferens from reserpine pretreated rats. During a 2 min interval, 45Ca influx induced by 140 mM K+ was identical to that produced by 35 mM K+. This suggests that a second mechanism may be involved in responses to high K+. Indeed, 140 mM K+ caused an approximately 300% increase above control in the formation of inositol trisphosphate (IP3) in tissues prelabelled with 3H-myoinositol whereas 35 mM K+ did not increase IP3. IP3 is thought to cause the release of calcium from internal stores which is consistent with our finding of an increase in 45Ca efflux into calcium-free medium from tissues prelabelled with 45Ca and stimulated with 140 mM K+. Stimulation with 35 mM K+ did not influence 45Ca efflux. We conclude that in the rat vas deferens high K+ promotes tension development by smooth muscle by a dual mechanism: influx of extracellular calcium and release of calcium from internal stores via an IP3 mechanism.

Animals↗
PubMed1987

Effects of potassium and norepinephrine on calcium influx in guinea-pig vas deferens.

The effects of potassium (K+) and adrenoceptor agonists on 45Ca influx and contraction of isolated vas deferens from reserpine-treated guinea-pigs has been investigated. K+ (18-84 mM) increased 45Ca influx with the maximum influx occurring at 50 mM K+. The rate of influx upon exposure to K+ was highest in the first 2 min and decreased thereafter. In contrast to K+, norepinephrine, methoxamine and xylazine did not increase the rate of 45Ca influx. Norepinephrine and methoxamine, but not xylazine, inhibited 45Ca influx induced by high K+. The inhibitory effect of norepinephrine was also present in denervated tissues. Prazosin, but not yohimbine or propranolol, blocked the inhibitory effect of norepinephrine. Potassium-induced contractions were significantly potentiated in the presence of norepinephrine, especially at low calcium concentrations. We conclude that K+ contracts the guinea-pig vas deferens by increasing the influx of calcium whereas stimulation of alpha 1-adrenoceptors prevents 45Ca influx elicited by high potassium.

Adrenergic alpha-Agonists↗
PubMed1985

Effect of postganglionic denervation and pretreatment with reserpine on alpha-adrenoceptors of the guinea-pig vas deferens.

The radiolabeled alpha-1-adrenoceptor antagonist [3H]WB-4101 was used to determine whether supersensitivity of the guinea pig vas deferens that develops after denervation or reserpine pretreatment is associated with a change in the characteristics of the alpha-1-adrenoceptor. Specific binding of [3H]WB-4101 to membrane fractions of vasa deferentia was saturable and reversible. The dissociation constant (KD) was similar when obtained by equilibrium or kinetic studies. As determined by Scatchard analysis, neither chronic (5 days) pretreatment with reserpine (1.0 mg/kg/day) nor postganglionic denervation (7 days) altered the number or affinity of [3H] WB-4101 binding sites. It is concluded that changes in alpha-1-receptor characteristics are not responsible for postjunctional supersensitivity in this tissue.

Adrenergic alpha-Antagonists↗