Excitatory motor innervation in the canine rectoanal region: role of changing receptor populations

1. Motor innervation in the canine rectoanal region was examined in isolated strips of the circular muscle layer. Contractile responses to electrical field stimulation began at lower frequencies and were more persistent in the internal anal sphincter (IAS) than in the rectum. 2. Motor innervation to the IAS was almost exclusively sympathetic, since it was blocked by guanethidine (Guan 3 microM) while the response in the proximal rectum was approximately 50% muscarinic, and sensitive to the M(3) selective antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 0.1 microM) and 50% tachykinergic, and sensitive to the neurokinin 2 (NK(2)) receptor antagonist GR 94800 (1 microM). From IAS to rectum there was a gradual shift in the relative contribution of intrinsic and extrinsic neural innervation. 3. Responses to exogenously applied transmitters exhibited a similar pattern to that observed with motor innervation. Norepinephrine (NE) was most potent in the IAS and acetylcholine (ACh) and NK-A were most potent in the proximal rectum. The responses were inhibited by prazosin, 4-DAMP and GR 94800 respectively. 4. A gradient in the density of adrenergic alpha(1), muscarinic and NK(2) receptors also existed from IAS to rectum as determined by measuring the binding of [(3)H]-prazosin, [(3)H]-quinuclidinyl benzilate ([(3)H]-QNB and [(3)H]-SR-48968 to smooth muscle membranes. 5. In summary, these data suggest that the shift in motor innervation in the rectoanal region is achieved in part by changes in receptor populations available for activation by sympathetic and enteric motor neurons.

Electrical activity induced by nitric oxide in canine colonic circular muscle

Nitric oxide generates slow electrical oscillations (SEOs) in cells near the myenteric edge of the circular muscle layer, which resemble slow waves generated by interstitial cells of Cajal (ICCs) at the submucosal edge of this muscle. The properties of SEOs were studied to determine whether these events are similar to slow waves. Rapid frequency membrane potential oscillations (MPOs; 16 +/- 1 cycles/min and 9.6 +/- 0.2 mV) were recorded from control muscles near the myenteric edge. Sodium nitroprusside (0.3 microM) reduced MPOs and initiated SEOs (1.3 +/- 0.3 cycles/min and 13.4 +/- 1.4 mV amplitude). SEOs were abolished by the guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (10 microM). MPOs were abolished by nifedipine (1 microM), whereas SEO frequency increased and the amount of depolarization decreased. BAY K 8644 (1 microM) prolonged SEOs and reduced their frequency. SEOs were abolished by Ni(2+) (0.5 mM), low Ca(2+) solution (0.1 mM Ca(2+)), cyclopiazonic acid (10 microM), and the mitochondrial uncouplers antimycin (10 microM) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM). Oligomycin (10 microM) was without effect. These effects are similar to those described for colonic slow waves. Our results suggest that nitric oxide-induced SEOs are similar in mechanism to slow waves, an activity not previously thought to be generated by myenteric pacemakers.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

beta-Adrenergic receptor stimulation of L-type Ca2+ channels in rabbit portal vein myocytes involves both alphas and betagamma G protein subunits

1. Previous studies have shown that purified G protein alphas and betagamma subunits stimulate vascular L-type Ca2+ channels through protein kinase A and C (PKA and PKC), respectively. The present study tested whether activation of endogenous G proteins via beta-adrenergic receptor binding also stimulates vascular Ca2+ channels through both Galphas and Gbetagamma and the subsequent activation of PKA and PKC. 2. Peak Ba2+ current (IBa) in freshly isolated rabbit portal vein smooth muscle cells was significantly increased by bath application of 0.5 microM isoproterenol (isoprenaline; ISO) when measured using the whole-cell patch clamp method (53 +/- 3 % increase, n = 15). Stimulation of IBa by ISO was partially reversed by a PKA inhibitor, KT 5720, or a PKC inhibitor, calphostin C, and completely blocked when cells were pretreated with both KT 5720 and calphostin C. 3. Dialysis of cells with polyclonal antibody to Galphas significantly reduced but did not completely eliminate ISO-induced stimulation of IBa. The remaining stimulation was abolished by calphostin C. Dialysis of cells with a polyclonal antibody to Gbeta also significantly reduced ISO-induced stimulation and the remaining stimulation was abolished by KT 5720. Dialysis of cells with both antibodies completely prevented the stimulation of IBa by ISO. 4. ISO-induced stimulation of IBa was reversed by ICI-118,551, a specific beta2-adrenoceptor antagonist, but not by CGP 20712A, a specific beta1-adrenoceptor antagonist. In addition, the beta2-adrenoceptor agonist zinterol significantly increased peak IBa while the beta1-adrenoceptor agonist dobutamine and beta3-adrenoceptor agonist BRL 37344A had little effect on peak IBa. 5. These data suggest that beta-adrenergic receptor stimulation of vascular L-type Ca2+ channels involves both alphas and betagamma G-protein subunits, which exert their effects through PKA and PKC, respectively.

Frequency dependent alpha(2)-adrenoceptor mediated modulation of excitatory junction potentials in guinea-pig mesenteric artery

Excitatory junctional potentials (EJPs) elicited with brief duration (10 s) electrical field stimulation of guinea-pig mesenteric arteries were nearly abolished at all frequencies by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 30 microM) but persisted following reserpinization. Suramin (100 microM) enhanced EJPs at 0.2-0.5 Hz responses and reduced them at 2-32 Hz. Phentolamine (1 microM) and yohimbine (0.1 microM) enhanced EJPs at 0.2-8 Hz but not at 16-32 Hz. Oxymetazoline (0.3 microM) reduced EJPs at 0.2-0.5 Hz but not at 1-32 Hz. Following reserpinization, EJPs were enhanced at 0.2-2 Hz but not at 4-32 Hz. Clonidine (0.1 microM) was without effect at all frequencies in control arteries but reduced EJPs at 0.2-2 Hz in reserpine-treated arteries. In conclusion, pre-junctional alpha(2)-adrenoceptors modulate ATP release during low frequency, brief duration sympathetic nerve stimulation.

Regulation of BK(Ca) channels expressed in human embryonic kidney 293 cells by epoxyeicosatrienoic acid

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites of cytochrome P450 monooxygenase, which are released from endothelial cells and dilate arteries. Dilation seems to be caused by activation of large-conductance Ca2+ activated K+ channels (BK(Ca)) leading to membrane hyperpolarization. Previous studies suggest that EETs activate BK(Ca) channels via ADP-ribosylation of the G protein Galphas with a subsequent membrane-delimited action on the channel [Circ Res 78:415-423, 1996; 80:877-884, 1997; 85:349-356, 1999]. The present study examined whether this pathway is present in human embryonic kidney (HEK) 293 cells when the BK(Ca) alpha-subunit (cslo-alpha) is expressed without the beta-subunit. 11,12-EET increased outward K+ current in whole-cell recordings of HEK293 cells. In cell-attached patches, 11,12-EET also increased the activity of cslo-alpha channels without affecting unitary conductance. This action was mimicked by cholera toxin. The ADP-ribosyltransferase inhibitors 3-aminobenzamide and m-iodobenxylguanidine blocked the stimulatory effect of 11,12-EET. In inside-out patches 11,12-EET was without effect on channel activity unless GTP was included in the bathing solution. GTP and GTPgammaS alone also activated cslo-alpha channels. Dialysis of cells with anti-Galphas antibody completely blocked the activation of cslo-alpha channels by 11,12-EET, whereas anti-Galphai/o and anti-Gbetagamma antibodies were without effect. The protein kinase A inhibitor KT5720 and the adenylate cyclase inhibitor SQ22536 did not reduce the stimulatory effect of 11,12-EET on cslo-alpha channels in cell-attached patches. These data suggest that EET leads to Galphas-dependent activation of the cslo-alpha subunits expressed in HEK293 cells and that the cslo-beta subunit is not required.

Cotransmission from sympathetic vasoconstrictor neurons: differences in guinea-pig mesenteric artery and vein

Vasoconstrictor responses to electrical field stimulation (EFS, 0.2-32 Hz, 0.1 ms, 12 V, for 1 min) were measured in endothelium-denuded segments of guinea-pig mesenteric vein and compared to responses in mesenteric artery. The distribution of both tyrosine-hydroxylase-like immunoreactivity (TH-LI) and neuropeptide Y-like immunoreactivity (NPY-LI) was also studied using anti-TH and anti-NPY antibodies. The effect of exogenous NPY (10 nM) on EFS (8 Hz, 0.3 ms, 12 V, for 1 min)-evoked overflow of noradrenaline (NA) was also studied using an HPLC technique with electrochemical detection. Veins responded with contractions at lower frequencies of stimulation than arteries. Prazosin (0.1 microM) abolished the EFS-evoked contractions in artery at 0.5-32 Hz and in vein at 0.2-1 Hz of stimulation. However, in vein, the contractile responses to EFS at 2-32 Hz of stimulation were only reduced by prazosin. Phentolamine (1 microM) abolished the responses to 0.5-4 Hz and reduced the responses to 8-32 Hz of EFS in artery. In vein, phentolamine (1 microM) abolished the responses to 0.2-1 Hz and facilitated the contractions elicited by 16-32 Hz. The NPY-receptor antagonist BIBP3226 (1 microM), in combination with phentolamine, abolished contractions in vein. Yohimbine (0.1 microM) abolished the responses to lower frequencies of stimulation in both artery (0.5-2 Hz) and vein (0.2-1 Hz). The responses to greater frequency stimulation were not affected by yohimbine in artery, and were facilitated in vein. Pre-treatment of animals for 24 h with reserpine abolished contractile responses to EFS in artery, whereas in vein, responses to 0.2-2 Hz were abolished while responses to 4-32 Hz were unchanged. Suramin (100 microM) or alpha,beta-methylene ATP (alpha,beta MeATP; 10-100 microM) treatment did not affect the contractile responses to EFS in either artery or vein. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS; 30 microM), even potentiated the responses to 2-16 Hz in vein. However, following resperine-treatment, both PPADS and suramin reduced the nerve-evoked contractions of vein. Either BIBP3226 (1 microM) alone or BIBP3226 in combination with PPADS or suramin abolished the contractile response to EFS in reserpine-treated veins. NPY (100 nM) produced significantly more contraction in vein than in artery (i.e., 93 +/- 2.5 versus 7 +/- 4% of the response to 70 mM KCl, respectively). NPY (10 nM) significantly reduced the NA overflow evoked by EFS at 8 Hz. Flat mount preparations and cryostat sections of both mesenteric artery and vein revealed that TH-LI and NPY-LI were co-localized in a dense network of fibers within the adventitial layer. In conclusion, NA exclusively mediates the contractile response to sympathetic nerve stimulation in guinea-pig mesenteric artery, whereas at least three neurotransmitters [i.e., NA, adenosine 5'-triphosphate (ATP) and NPY] are involved in the neural response of mesenteric vein.

Multiple P2Y receptors mediate contraction in guinea pig mesenteric vein

Vasoconstrictor responses to exogenous adenine and pyrimidine nucleotides were measured in endothelium-denuded segments of guinea pig mesenteric vein and compared with responses in mesenteric artery. The rank order of potency for nucleotides in veins was: 2-MeSADP = 2-MeSATP > UTP > ATPgammaS = alpha,betaMeATP > UDP = ATP > ADP >> beta,gamma-D-MeATP = beta,gamma-L-MeATP. In contrast 2-MeSADP, UTP, and UDP were inactive in arteries, and the rank order of potency of other nucleotides differed; that is, alpha,betaMeATP > beta, gamma-D-MeATP > beta,gamma-L-MeATP = ATPgammaS = 2-MeSATP > ATP > ADP. In veins, UTP, ATP, and 2-MeSATP were more efficacious contractile agents than alpha,beta MeATP. In addition, the ability to desensitize responses to these nucleotides and inhibit them with various blockers differed. The response to alpha,betaMeATP in veins exhibited rapid desensitization and was inhibited by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS) and suramin. The response to 2-MeSATP in veins did not desensitize; nor was it inhibited by prior alpha,betaMeATP desensitization, but it was inhibited by PPADS, suramin, and the selective P2Y(1) receptor antagonist adenosine 3',5'-bisphosphate (ABP, 10-100 microM). Responses to ATP and UTP in veins did not desensitize and were not inhibited by PPADS, suramin, ABP, or alpha, betaMeATP desensitization. In conclusion, our results suggest that venous contraction to a variety of nucleotides is mediated in large part by P2Y receptors including P2Y(1) receptors and an UTP-preferring P2Y receptor. A small component of contraction also appears to be mediated by P2X(1) receptors. This receptor profile differs markedly from that of mesenteric arteries in which P2X(1) receptors predominate.

An unusual Ca(2+) entry pathway activated by protein kinase C in dog splenic artery

The characteristics of the Ca(2+) entry pathways that are activated by protein kinase C (PKC) in canine splenic artery were investigated. Phorbol 12, 13-dibutyrate (PDB) contracted tissues and increased Ca(2+) influx. PDB-induced contraction was reduced by preincubation of tissues in Ca(2+)-free Krebs' solution (1 mM EGTA) but was unaffected when Ca(2+)-free solution was applied after contraction was initiated with PDB. In contrast, (45)Ca influx and contraction induced by PDB were resistant to nifedipine, Cd(2+), Gd(3+), La(3+), or Ni(2+) whether added before or during exposure to PDB. Indeed, Cd(2+) reduced (45)Ca(2+) efflux and potentiated Ca(2+) influx, but not PDB-induced contraction. Norepinephrine (NE)-induced contractions were inhibited by preincubation in Ca(2+)-free Krebs' solution (1 mM EGTA). Nifedipine (10 microM) led to a small reduction in the NE-induced contraction but was without effect on (45)Ca(2+) influx. Pretreatment for 16 min with Cd(2+), Gd(3+), or La(3+) (each 1 mM) reduced or abolished NE-induced contraction and Ca(2+) influx. Application of these cations after exposure to NE did not affect (45)Ca(2+) influx but reduced tension. The Q(10) for the increase in (45)Ca(2+) influx was approximately 2 for high K(+) and NE, but 4 for PDB. The results suggest that stimulation of PKC in dog splenic artery activates a Ca(2+) entry pathway that is resistant to di- and trivalent cations. The inhibition of Ca(2+) influx by preincubating with cations during short-term exposure to NE may represent an action on Ca(2+) turnover that precedes activation of PKC.

Anchoring protein is required for cAMP-dependent stimulation of L-type Ca(2+) channels in rabbit portal vein

Stimulation of cardiac L-type Ca(2+) channels by cAMP-dependent protein kinase (PKA) requires anchoring of PKA to a specific subcellular environment by A-kinase anchoring proteins (AKAP). This study evaluated the possible requirement of AKAP in PKA-dependent regulation of L-type Ca(2+) channels in vascular smooth muscle cells using the conventional whole cell patch-clamp technique. Peak Ba(2+) current in freshly isolated rabbit portal vein myocytes was significantly increased by superfusion with either 0.5 microM isoproterenol (131 +/- 3% of the control value, n = 11) or 10 microM 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP; 114 +/- 1%, n = 8). The PKA-induced stimulatory effects of both isoproterenol and 8-BrcAMP were completely abolished by a specific PKA inhibitor KT-5720 (0.2 microM) or by dialyzing cells with Ht 31 (100 microM), a peptide that inhibits the binding of PKA to AKAP. In contrast, Ht 31 did not block the excitatory effect of the catalytic subunit of PKA when dialyzed into the cells. These data suggest that stimulation of Ca(2+) channels in vascular myocytes by endogenous PKA requires localization of PKA through binding to AKAP.

Regulation of L-type Ca2+ channels in rabbit portal vein by G protein alphas and betagamma subunits

1. The effect of purified G protein subunits alphas and betagamma on L-type Ca2+ channels in vascular smooth muscle and the possible pathways involved were investigated using freshly isolated smooth muscle cells from rabbit portal vein and the whole-cell patch clamp technique. 2. Cells dialysed with either Galphas or Gbetagamma exhibited significant increases in peak Ba2+ current (IBa) density (148 % and 131 %, respectively) compared with control cells. The combination of Galphas and Gbetagamma further increased peak IBa density (181 %). Inactive Galphas and Gbetagamma did not have any effect on Ca2+ channels. 3. The stimulatory effect of Galphas on peak IBa was entirely abolished by the protein kinase A inhibitor Rp-8-Br-cAMPS, or the adenylyl cyclase inhibitor SQ 22536. On the other hand, the stimulatory response of Ca2+ channels to Gbetagamma was not affected by the protein kinase A inhibitors Rp-8-Br-cAMPS and KT 5720, or by the Ca2+-dependent protein kinase C inhibitor bisindolylmaleimide 1, but was completely blocked by the protein kinase C inhibitor calphostin C. Pretreatment of cells with phorbol 12-myristate 13-acetate for over 18 h prevented the stimulatory effect of Gbetagamma on peak IBa. In addition, acute application of phorbol 12,13-dibutyrate enhanced peak IBa density in control cells, which could be entirely blocked by calphostin C. 4. These data indicate that enhancement of Ba2+ currents by Galphas and Gbetagamma can be attributed to increased activity of protein kinase A and protein kinase C, respectively. No direct membrane-delimited pathway for Ca2+ channel regulation by activated Gs proteins could be detected in vascular smooth muscle cells.

Cyclic GMP-dependent protein kinase activates cloned BKCa channels expressed in mammalian cells by direct phosphorylation at serine 1072

NO-induced activation of cGMP-dependent protein kinase (PKG) increases the open probability of large conductance Ca2+-activated K+ channels and results in smooth muscle relaxation. However, the molecular mechanism of channel regulation by the NO-PKG pathway has not been determined on cloned channels. The present study was designed to clarify PKG-mediated modulation of channels at the molecular level. The cDNA encoding the alpha-subunit of the large conductance Ca2+-activated K+ channel, cslo-alpha, was expressed in HEK293 cells. Whole cell and single channel characteristics of cslo-alpha exhibited functional features of native large conductance Ca2+-activated K+ channels in smooth muscle cells. The NO-donor sodium nitroprusside increased outward current 2.3-fold in whole cell recordings. In cell-attached patches, sodium nitroprusside increased the channel open probability (NPo) of cslo-alpha channels 3.3-fold without affecting unitary conductance. The stimulatory effect of sodium nitroprusside was inhibited by the PKG-inhibitor KT5823. Direct application of PKG-Ialpha to the cytosolic surface of inside-out patches increased NPo 3.2-fold only in the presence of ATP and cGMP without affecting unitary conductance. A point mutation of cslo-alpha in which Ser-1072 (the only optimal consensus sequence for PKG phosphorylation) was replaced by Ala abolished the PKG effect on NPo in inside-out patches and the effect of SNP in cell attached patches. These results indicate that PKG activates cslo-alpha by direct phosphorylation at serine 1072.

Parallel pathways mediate inhibitory effects of vasoactive intestinal polypeptide and nitric oxide in canine fundus

1. The gastric adaptation reflex is activated by the release of non-adrenergic, non-cholinergic (NANC) inhibitory transmitters, including nitric oxide (NO) and vasoactive intestinal polypeptide (VIP). The role of NO in this reflex is not disputed, but some investigators suggest that NO synthesis is stimulated by VIP in post-junctional cells or in nerve terminals. We investigated whether the effects of these transmitters are mediated by independent pathways in the canine gastric fundus. 2. VIP and NO produced concentration-dependent relaxation of the canine fundus. Nomega-nitro-L-arginine (L-NNA) reduced relaxation induced by electrical field stimulation (EFS; 0.5-8 Hz), but had no effect on responses to exogenous VIP and sodium nitroprusside (SNP, 10 microM). 3. Oxyhaemoglobin reduced relaxations produced by EFS and SNP. Oxyhaemoglobin also reduced relaxation responses to low concentrations of VIP (<10 nM), but these effects were non-specific and mimicked by methaemoglobin which had no effect on nitrergic responses. 4. A blocker of guanylyl cyclase, 1H-[1,2,4]oxidiazolo [4,3,-a]quinoxalin-1-one, (ODQ) inhibited responses to EFS, SNP and DETA/NONOate (an NO.donor), but had no effect on responses to VIP. cis-N-(2-phenylcyclopentil)-azacyclotridec-1en-2-amine monohydrochloride (MDL 12,330A), a blocker of adenylyl cyclase, reduced responses to EFS, VIP and forskolin, but did not affect responses to SNP. 5. Levels of cyclic GMP were enhanced by the NO donor S-nitroso-n-acetylpenicillamine (SNAP) but were unaffected by VIP (1 microM). The increase in cyclic GMP in response to SNAP was blocked by ODQ. 6. The results suggest that at least two transmitters, possibly NO and VIP, mediate relaxation responses in the canine fundus. NO and VIP mediate responses via cyclic GMP- and cyclic AMP-dependent mechanisms, respectively. No evidence was found for a serial cascade in which VIP is coupled to NO-dependent responses.

Modulation of cholinergic neuromuscular transmission by nitric oxide in canine colonic circular smooth muscle

This study examines the effect of nitric oxide (NO) on cholinergic transmission in strips of canine colonic circular muscle in which neural plexus-pacemaker regions had been removed. Electrical field stimulation gave rise to atropine- and TTX-sensitive excitatory junction potentials (EJPs), the amplitude of which were frequency dependent. In 47% of control muscles, the EJP was followed by an inhibitory junction potential (IJP), whereas in the presence of atropine all preparations exhibited only IJPs. The NO synthase inhibitor Nomega-nitro-L-arginine (L-NNA), the guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (ODQ), and the protein kinase G (PKG) antagonist Rp-8-bromo-PET-cGMPS all significantly increased EJP amplitude and reduced or abolished IJPs. The potentiation of EJPs by L-NNA was reversed by the NO donors sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine in a manner blocked by ODQ. [14C]ACh overflow was also measured to evaluate the possible prejunctional effects of NO. Both norepinephrine and TTX significantly decreased [14C]ACh overflow; however, L-NNA, ODQ, and SNP were without effect. These data suggest that both cholinergic and nitrergic motoneurons functionally innervate the interior of the circular muscle layer. The inhibitory actions of NO on cholinergic transmission appear to be post- rather than prejunctional and to involve guanylyl cyclase as well as possibly PKG.

Basal activation of ATP-sensitive potassium channels in murine colonic smooth muscle cell

The function and molecular expression of ATP-sensitive potassium (KATP) channels in murine colonic smooth muscle was investigated by intracellular electrical recording from intact muscles, patch-clamp techniques on isolated smooth muscle myocytes, and reverse transcription polymerase chain reaction (RT-PCR) on isolated cells. Lemakalim (1 microM) caused hyperpolarization of intact muscles (17. 2 +/- 3 mV). The hyperpolarization was blocked by glibenclamide (1-10 microM). Addition of glibenclamide (10 microM) alone resulted in membrane depolarization (9.3 +/- 1.7 mV). Lemakalim induced an outward current of 15 +/- 3 pA in isolated myocytes bathed in 5 mM external K+ solution. Application of lemakalim to cells in symmetrical K+ solutions (140/140 mM) resulted in a 97 +/- 5 pA inward current. Both currents were blocked by glibenclamide (1 microM). Pinacidil (1 microM) also activated an inwardly rectifying current that was insensitive to 4-aminopyridine and barium. In single-channel studies, lemakalim (1 microM) and diazoxide (300 microM) increased the open probability of a 27-pS K+ channel. Openings of these channels decreased with time after patch excision. Application of ADP (1 mM) or ATP (0.1 mM) to the inner surface of the patches reactivated channel openings. The conductance and characteristics of the channels activated by lemakalim were consistent with the properties of KATP. RT-PCR demonstrated the presence of Kir 6.2 and SUR2B transcripts in colonic smooth muscle cells; transcripts for Kir 6.1, SUR1, and SUR2A were not detected. These molecular studies are the first to identify the molecular components of KATP in colonic smooth muscle cells. Together with the electrophysiological experiments, we conclude that KATP channels are expressed in murine colonic smooth muscle cells and suggest that these channels may be involved in dual regulation of resting membrane potential, excitability, and contractility.

Control of motility patterns in the human colonic circular muscle layer by pacemaker activity

1. This study characterized the electrical and mechanical activities of human colonic muscle strips obtained from either the ascending, descending or sigmoid colon of patient volunteers during elective colon resections. 2. Rhythmic contractile activity was observed in colonic circular muscle strips in the absence of external stimuli. This activity persisted in the presence of atropine, phentolamine, propranolol, tetrodotoxin and Nomega-nitro-L-arginine but was abolished by nifedipine. 3. The activity of whole circular muscle (WCM) was compared with that of the myenteric half (MCM), the submucosal half (SCM) and the interior (ICM) of the circular muscle layer. WCM exhibited a prominent 2-4 contractions min-1 contractile pattern which was also present in strips of SCM. In contrast, MCM and ICM exhibited slow (0.3-0.6 contractions min-1), long duration contractions with superimposed higher frequency contractions (17-18 contractions min-1). 4. Resting membrane potential (Vm), recorded at various positions through the thickness of WCM strips did not differ and averaged -50 mV. 5. Slow waves were observed in 83 % of muscles. They averaged 12 mV in amplitude, 9.4 s in duration and had a frequency of 2-4 contractions min-1. Slow waves were greatest in amplitude near the submucosal edge and decreased with distance away from this edge. Each slow wave was associated with a transient contraction. 6. Near the myenteric edge, rapid fluctuations of Vm with a mean frequency of 18 contractions min-1 were recorded in 67 % of muscles. Spiking activity was common and was superimposed upon slow waves and rapid Vm fluctuations. 7. In summary, slow waves were identified in the human colonic circular muscle layer which arise at or near the submucosal edge. These electrical events give rise to a 2-4 contractions min-1 contractile rhythm which is characteristic of the intact muscle layer. Thus, the nature and spatial organization of pacemaker activity in the human colon bears significant resemblance to other animal models, such as the dog and pig.

Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

1. Acetylcholine (ACh) elicits an endothelium-dependent relaxation and hyperpolarization in the absence of nitric oxide (NO) and prostaglandin synthesis in the guinea-pig coronary artery (GPCA). This response has been attributed to a factor termed endothelial-derived hyperpolarizing factor (EDHF). Recently it has been suggested that EDHF may be a cytochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatrienoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the presence of 100 microM N(omega)-nitro-L-arginine and 10 microM indomethacin. 2. ACh, AA and 11,12-EET each produced concentration-dependent relaxations in arteries contracted with the H1-receptor agonist AEP (2,2-aminoethylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatetranynoic acid (30 microM). 3. The cytochrome P450 inhibitors proadifen (10 microM) and clotrimazole (10 microM) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 microM) and 7-ethoxyresorufin (10 microM) were without effect on all three vasodilators. Proadifen and clotrimazole also inhibited ACh (1 microM) and LEM (1 microM)-induced hyperpolarization. 4. The ability of various potassium channel blockers to inhibit relaxation responses elicited with ACh, AA and 11,12-EET was also determined. Iberiotoxin (IBTX; 100 nM) was without effect on responses to ACh but significantly reduced responses to both AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) significantly reduced response to ACh but not responses to AA and 11,12-EET. Combined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a greater extent than 4-AP alone. Apamin (1 microM), glibenclamide (10 microM) and BaCl2 (50 microM) had no significant effect on responses to ACh, AA and 11,12-EET. 5. IBTX (100 nM) significantly reduced both 11,12-EET (33 microM) and AA (30 microM) hyperpolarization without affecting the ACh (1 microM)-induced hyperpolarization. In contrast, 4-AP significantly reduced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations. 6. In summary, our results suggest that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacology between AA and 11,12-EET suggests that this factor is an EET. However, the disparity of pharmacology between responses to ACh versus responses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that leads to NO- and prostaglandin-independent hyperpolarization and relaxation in the GPCA.

Modulation of Ca2+ channels by cyclic nucleotide cross activation of opposing protein kinases in rabbit portal vein

Cyclic nucleotides are known to modify voltage-gated (L-type) Ca2+ channel activity in vascular smooth muscle cells, but the exact mechanism(s) underlying these effects is not well defined. The purpose of the present study was to investigate the modulatory role of the cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) pathways in Ca2+ channel function by using both conventional and perforated-patch-clamp techniques in rabbit portal vein myocytes. The membrane-permeable cAMP derivative, 8-bromo cAMP (0.1 to 10 micromol/L), significantly increased (14% to 16%) peak Ba2+ currents, whereas higher concentrations (0.05 to 0.1 mmol/L) decreased Ba2+ currents (23% to 31%). In contrast, 8-bromo cGMP inhibited Ba2+ currents at all concentrations tested (0.01 to 1 mmol/L). Basal Ca2+ channel currents were significantly inhibited by the PKA blocker 8-Bromo-2'-O-monobutyryladenosine-3',5'-monophosphorothioate, Rp-isomer (Rp 8-Br-MP cAMPS, 30 micromol/L) and enhanced by the PKG inhibitor beta-Phenyl-1,N2-etheno-8-bromoguanosine-3',5'-monophosphorothioate, Rp-isomer (Rp-8-Br PET cGMPS, 10 nmol/L). In the presence of Rp 8-bromo PET cGMPS (10 to 100 nmol/L), both 8-bromo cAMP (0.1 mmol/L) and 8-bromo cGMP (0.1 mmol/L) enhanced Ba2+ currents (13% to 39%). The excitatory effect of 8-bromo cGMP was blocked by Rp 8-bromo MB-cAMPS. Both 8-bromo cAMP (0.05 mmol/L) and forskolin (10 micromol/L) elicited time-dependent effects, including an initial enhancement followed by suppression of Ba2+ currents. Ba2+ currents were also enhanced when cells were dialyzed with the catalytic subunit of PKA. This effect was reversed by the PKA blocker KT 5720 (200 nmol/L). Our results suggest that cAMP/PKA stimulation enhances and cGMP/PKG stimulation inhibits L-type Ca2+ channel activity in rabbit portal vein myocytes. Our results further suggest that both cAMP and cGMP have a primary action mediated by their own kinase as well as a secondary action mediated by the opposing kinase.

Adenosine-induced hyperpolarization in guinea pig coronary artery involves A2b receptors and KATP channels

The role of P1 purinoceptor subtypes, the adenylyl cyclase (AC) pathway, and ATP-sensitive K+ (KATP) channels in adenosine (Ado)-induced membrane hyperpolarization was investigated in isolated segments of the guinea pig coronary artery using conventional microelectrode techniques. Ado (1-100 microM) elicited concentration-dependent hyperpolarization (half-maximal effective concentration 7.5 +/- 0.5 microM) that averaged 28.6 +/- 2.9 mV at 100 microM Ado. The A1 selective agonist N6-cyclopentyladenosine (CPA), the A1/A2 agonist 2-chloroadenosine, and the A2a/A2b agonist 5-(N-ethylcarboxamido)adenosine (NECA) each induced glibenclamide (3 microM)-sensitive hyperpolarization at 10 microM. However, the selective A2a-receptor agonists CGS-21680 and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methoxyphenyl]ethyladenosine (10 microM each) were without effect. Responses to CPA and NECA were significantly reduced by the AC inhibitor SQ-22,536 (100 microM). Activation of the AC-adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) pathway by four additional methods, i.e., 1) forskolin (0.3-1 microM), 2) isoproterenol (0.1-1 microM), 3) combined milrinone (0.4 microM) and rolipram (30 microM), and 4) combined N6-phenyladenosine 3',5'-monophosphate, 8-(6-aminohexyl)aminoadenosine 3',5'-cyclic monophosphate, and the Sp-isomer of 5,6-dichloro-1-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphothioate (100 microM each), also gave rise to glibenclamide-sensitive hyperpolarization. These results suggest that stimulation of A2b receptors coupled to AC represents the predominant mechanism by which Ado elicits hyperpolarization in this vessel. The ensuing increase in cAMP activates PKA, which then increases the activity of KATP channels. Our results further suggest that KATP channels are an important target for numerous pathways that raise cAMP levels in the coronary artery.

Characterization of delayed rectifier K+ currents in rabbit coronary artery cells near resting membrane potential

Previous studies have suggested that delayed rectifier K+ (Kdr) channels contribute to the control of membrane potential in vascular smooth muscle. To explore this hypothesis further, we investigated the characteristics of Kdr channels in the negative voltage range in the rabbit coronary artery. The Kdr channel blocker 4-aminopyridine (1 mM) contracted intact vessels and depolarized them from -52 to -37 mV, suggesting that these channels significantly contribute to the maintenance of resting membrane potential. In contrast, the ATP-sensitive K+ channel blocker glybenclamide (3 microM) had little effect on resting tone and did not alter the contraction elicited with 4-aminopyridine. K+ currents in isolated cells were then investigated by using whole-cell patch-clamp techniques. Increasing extracellular K+ concentration ([K+]o) from 5 to 135 mM resulted in the appearance of large inward currents at potentials between -60 and 0 mV. The voltage dependence of conductance for inward K+ currents was steeper and shifted toward more negative potentials when compared with outward K+ currents in 5 mM [K+]o solution. Various blockers of Kdr channels, i.e., 4-aminopyridine (3 mM), phencyclidine (0.1 mM), and intracellular tetraethylammonium (10 mM), nearly abolished currents in high [K+]o solution. In contrast, Ba2+ (0.1 mM) was without effect. These results suggest that the inward currents detected at potentials between -60 and 0 mV in high [K+]o solution are Kdr currents. Our results suggest that Kdr channels physiologically contribute to the control of membrane potential in the rabbit coronary artery.

Basal release of nitric oxide induces an oscillatory motor pattern in canine colon

1. The consequences of intrinsic, basal nitric oxide release on electrical and contractile activity of canine proximal colon were examined. Membrane potential and contraction were simultaneously recorded from the circular muscle in the presence of drugs to block adrenergic and cholinergic responses. 2. Electrical slow waves were recorded from muscle cells near the submucosal surface of the circular layer. Spontaneous contractions were initiated by each slow wave. Contractile amplitude increased 1.9-fold when nerves were blocked with tetrodotoxin (TTX, 1 microM). 3. Muscle cells near the myenteric surface displayed myenteric potential oscillations (MPOs) averaging 16 cycles per minute (c.p.m.) in frequency and 10 mV in amplitude. Twenty-five per cent of muscles displayed an additional slow, neurogenic oscillation (mean frequency, 1 c.p.m.; amplitude, 14 mV) superimposed upon the MPO rhythm. 4. The nitric oxide (NO) synthase inhibitor N omega -nitro-L-arginine (L-NA, 100 microM; n = 16) abolished neurogenic oscillations, depolarized cells, and increased MPO upstroke velocity, amplitude and frequency. The actions of L-NA were mimicked by N omega-nitro-L-arginine methylester (L-NAME, 100 microM) and oxyhaemoglobin (3%). 5. Spontaneous contractions were increased 2.3-fold by L-NA, and TTX had no effect on contractions after addition of L-NA. 6. The NO-donor sodium nitroprusside (SNP, 1 microM) reversed the electrical and mechanical effects of L-NA and initiated slow oscillations similar to the neurogenic oscillations. Slow oscillations were also evoked with S-nitroso-N-acetylpenicillamine (SNAP, 1 microM). The effects of NO donors were blocked by oxyhaemoglobin. 7. Slow electrical oscillations could not be elicited by SNP after removal of a thin strip of circular muscle along the myenteric edge. 8. These data suggest that the spontaneous electrical and contractile activity of the proximal colon is tonically suppressed by basal release of NO. Basal NO causes an oscillatory pattern of electrical and mechanical activity. This activity does not require patterned firing of nerves; rather a continuous, low level release of NO would be capable of producing the neurogenic oscillatory behaviour. The slow oscillatory activity depends upon the presence of the myenteric region of the circular muscle layer, which contains cell bodies of enteric neurons and interstitial cells of Cajal.

Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens

The present study investigates how changes in intracellular Ca2+ concentration modulate the influx of 45Ca2+ in isolated rat vasa deferentia. Raising extracellular K+ concentration ([K+]0) to > or = 32 mM increased 45Ca2+ influx during the 1st min in solutions containing 0.03-1.5 mM extracellular Ca2+ concentration ([Ca2+]0). During the 6th min in [K+]0 > or = 50 mM, 45Ca2+ influx was less than during the 1st min. This decline in 45Ca2+ influx occurred for [Ca2+]0 > or = 0.4 mM. Procaine potentiated K(+)-stimulated 45Ca2+ influx in 1.5 mM [Ca2+]0 and eliminated the decline of 45Ca2+ influx in low [Ca2-]0. Ryanodine and norepinephrine reduced K(+)-stimulated 45Ca2+ influx. 45Ca2+ content changed with time in accordance with the changes observed in 45Ca2+ influx. In isolated cells, voltage-dependent inward currents inactivated more rapidly with 1.5 mM Ca2+ as the charge carrier than with 1.5 mM Ba2+, and the steady-state inactivation relationship was shifted in the hyperpolarizing direction. Inward current was reduced with either caffeine, ryanodine, or norepinephrine. The inhibitory effects of norepinephrine were abolished by depletion of intracellular Ca2+ stores. These results are compatible with the hypothesis that K(+)-stimulated 45Ca2+ influx declines with time due to Ca(2+)-induced inhibition of Ca2- channels. Ca(2+)- and inositol 1,4,5-trisphosphate-induced releases of Ca2+ from the sarcoplasmic reticulum appear to play an important role in this process.

Relationship between nitric oxide and vasoactive intestinal polypeptide in enteric inhibitory neurotransmission

Although considerable evidence suggests that NO serves as a neurotransmitter in gastrointestinal muscles, it is unlikely to be the only substance involved in enteric inhibitory neurotransmission. Vasoactive intestinal polypeptide (VIP) is known to be expressed by inhibitory motor neurons in the gut, and it appears to be co-localized with nitric oxide synthase (NOS) in a subpopulation of enteric neurons. These data suggest that NO and VIP may be parallel neurotransmitters. Others have suggested that VIP is the primary inhibitory transmitter, and it stimulates production of NO in smooth muscle cells. In this "serial cascade" model NO is a paracrine substance. We performed experiments on circular muscles and cells from the canine proximal colon to further test the idea that NO and VIP are parallel neurotransmitters and to determine the validity of the serial cascade model in these muscles. We found that NO-independent inhibitory effects were unmasked when excitatory and NO-dependent inhibitory responses were blocked. NO-independent inhibitory effects were reduced by alpha-chymotrypsin and blocked by tetrodotoxin. NOS- and VIP-like immunoreactivities were co-localized in enteric neurons and varicose fibers in the circular muscle layer. Similar to several other reports we found no evidence for a constitutive NOS in smooth muscle cells. Several aspects of the serial cascade model were not supported by our results: (i) the electrical and mechanical effects of VIP did not depend upon NO synthesis; (ii) VIP-induced changes in [Ca2+]i did not depend upon NO synthesis; and (iii) VIP did not cause the release of NO from canine colonic muscles. These results are consistent with the hypothesis that NO and VIP are co-transmitters, released in parallel from enteric inhibitory nerves.