Expression of nitric oxide synthase immunoreactivity by interstitial cells of the canine proximal colon

A subpopulation of interstitial cells (ICs) are interposed between nerve terminals and smooth muscle cells in the gastrointestinal tract and may participate in neuromuscular transmission. These cells appear to be targets for NO released from enteric inhibitory nerves and respond to exogenous NO with: (i) an elevation in cGMP levels; (ii) an increase in intracellular Ca2+; (iii) and release of a diffusible substance that has tentatively been identified as NO. For the latter to be possible, ICs must express a constitutive isoform of NOS. This study characterized the expression of NOS-like immunoreactivity (NOS-LI) in ICs of the canine colon using 3 antibodies raised against the 2 known constitutive forms of NOS (i.e., neural (nNOS) and endothelial (eNOS) isoforms). Antibodies raised against cNOS and an antibody raised against rat cerebellar nNOS labeled ICs along the submucosal surface of the circular muscle layer (IC-SM), along the surface of septa that separate the circular muscle into fiber bundles (IC-SM), and in the myenteric region between the circular and longitudinal muscle layers (IC-MY). Another antibody raised against rat cerebellar nNOS failed to label ICs. Cultured IC-SM also expressed NOS-LI, suggesting that this feature of the IC phenotype survives culture conditions. Arteriolar endothelial cells in the canine colon were labeled with the same 2 antibodies that labeled ICs, suggesting there are significant structural similarities between NO synthases in ICs and endothelial cells. The data suggest that IC-SM and IC-MY express a constitutive form of NOS. Synthesis of NO by ICs may influence electrical rhythmicity and may serve to amplify and even propagate enteric inhibitory neurotransmission.

Dorsal root ganglion neurons of embryonic chicks contain nitric oxide synthase and respond to nitric oxide

We investigated the function of nitric oxide (NO) in dorsal root ganglion (DRG) neurons from 10 day embryonic chicks and adult birds. NADPH-diaphorase activity, a histochemical marker for nitric oxide synthase (NOS) in paraformaldehyde-fixed neurons, and NOS-like immunoreactivity were localized in all neurons in thoracic and lumbar ganglia from embryos. However, only a subset of neurons from adults contained NOS-like immunoreactivity and NADPH-diaphorase activity. Thus, embryonic chick DRG neurons have the potential to synthesize NO in response to elevated cytoplasmic Ca2+. We also investigated the ability of dissociated embryonic chick DRG neurons to respond to NO by examining the effects of NO donors and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) on Ca2+ current (ICa) using the amphotericin-permeabilized patch-clamp technique: sodium nitroprusside (5 microM) reduced ICa to 0.68 +/- 0.06 (mean +/- S.D., n = 5) of control, S-nitroso-N-acetylpenicillamine (1 microM) reduced ICa to 0.44 +/- 0.06 (n = 4) of control, while 8-Br-cGMP (1 mM) reduced ICa to 0.58 +/- 0.22 (n = 5) of control. ICa was reduced in every neuron tested and this effect was partially reversed after approximately 10 min of washing. Thus, ICa of embryonic chick DRG neurons is inhibited by NO, possibly by a cGMP-dependent mechanism. These results indicate that all DRG neurons in embryonic chicks contain NOS-like immunoreactivity and respond to NO. Further, the percentage of NADPH-diaphorase positive neurons is reduced during development.

Localization of nitric oxide synthase in canine ileocolonic and pyloric sphincters

The distribution of neurons containing NAD-PH-diaphorase (NADPH-d) activity and nitric oxide synthase-like immunoreactivity (NOS-LI) in the canine pyloric and ileocolonic sphincters was studied. Cells within the myenteric and submucosal ganglia were positive for NADPH-d. These cells generally had the morphology of Dogiel type-I enteric neurons, however, there was some diversity in the morphology of NADPH-d-positive neurons in the myenteric plexus of the pylorus. Intramuscular ganglia were observed in both sphincters, and NADPH-d was found in a sub-population of neurons within these ganglia. Dual staining with an antiserum raised against nitric oxide synthase (NOS) demonstrated that almost all cells with NOS-LI were also NADPH-d positive. Varicose fibers within ganglia and within the circular and longitudinal muscle layers also possed NOS-LI and NADPH-d activity. Dual staining with anti-VIP antibodies showed that some of the NADPH-d-positive cells in the myenteric and submucosal ganglia also contained VIP-LI, but all VIP-LI-positive cells did not express NADPH-d activity. These data are consistent with recent physiological studies suggesting that nitric oxide serves as an inhibitory neurotransmitter in the pyloric and ileocolonic sphincters. The data also suggest that VIP is expressed in a sub-population of NADPH-d-positive neurons and may therefore act as a co-transmitter in enteric inhibitory neurotransmission to these specialized muscular regions.

Characterization of the properties of canine colonic smooth muscle in culture

We have developed and characterized an organ culture system that maintains the viability of colonic smooth muscles. Morphological, mechanical, electrical, and molecular properties of cultured canine colonic circular muscles were determined. Strips of circular muscle were cultured for up to 6 days. The smooth muscle phenotype was retained during culture; muscles contracted to agonists and responded to electrical field stimulation, suggesting that intrinsic nerves also survived in culture. Morphological analysis showed identifiable smooth muscle cells, enteric neurons, and interstitial cells, but some alterations in ultrastructure were also observed. Mechanical responses to acetylcholine suggested that the muscles developed supersensitivity during the culture period. The resting membrane potentials of cells near the submucosal surface of the circular muscle layer decreased from -82 mV on day 0 to -55 mV on day 3. Similar changes in the resting potential gradient occur when colonic muscles are treated with inhibitors of the Na(+)-K(+)-ATPase. Resting potentials of day 3 muscles remained constant in low external K+ (0.1 mM), suggesting little contribution of the pump to resting potential. Northern analysis of RNA from muscles cultured up to 6 days showed that the alpha 2-isoform of the pump decreased. The data suggest that organ-cultured strips of smooth muscle may provide a useful tool for evaluating electrical and mechanical events in conjunction with molecular analysis of functional components.

Enteric inhibitory neural regulation of human colonic circular muscle: role of nitric oxide

BACKGROUND

Nitric oxide and an apamin-sensitive transmitter may both contribute to neural inhibition in the human colon. The present study investigated the role of NO in regulating spontaneous rhythmic contractions and examined NO-dependent and independent components of neurally evoked hyperpolarization in the human colon.

METHODS

Mechanical and electrical activity were recorded from isolated circular muscle strips.

RESULTS

Rhythmic contractions were inhibited by nerve stimulation. This response was reduced by apamin, oxyhemoglobin, and L-NG-nitro arginine methyl ester (L-NAME). Electrical recording revealed two components of neurally evoked hyperpolarization: a fast hyperpolarization resulting from a single stimulus and a sustained hyperpolarization that developed with repetitive stimulation. Fast hyperpolarization was not affected by L-NAME or oxyhemoglobin but was significantly reduced by apamin. The sustained hyperpolarization was reduced by L-NAME or apamin. Exogenous NO and the P2y receptor agonist 2-methylthio adenosine 5'-triphosphate (2-MATP) inhibited spontaneous contractions and produced hyperpolarization. Apamin reduced the effects of 2-MATP but not those of NO.

CONCLUSIONS

The results support the concept that the inhibitory neurotransmission in the human colon involves two transmitters. A single stimulus results in an apamin-sensitive response. With multiple stimuli, a NO-dependent response develops and sums with the apamin-sensitive mechanism, producing sustained hyperpolarization and inhibition of contractions.

Immunohistochemical localization of 3',5'-cyclic guanosine monophosphate in the canine proximal colon: responses to nitric oxide and electrical stimulation of enteric inhibitory neurons

There is growing evidence that nitric oxide serves as a neurotransmitter released from enteric inhibitory nerves in the gastrointestinal tract. The distribution of nitric oxide synthase suggests that nitric oxide may also be a neurotransmitter within enteric ganglia. Since many actions of nitric oxide are mediated by stimulation of soluble guanylate cyclase and a subsequent increase in 3',5'-cyclic guanosine monophosphate (cGMP) concentration, targets for nitric oxide in the canine proximal colon were investigated by immunohistochemical localization of cGMP. In the presence of phosphodiesterase inhibitors (M&B 22948, 100 microM and 3-isobutyl-1-methyl-xanthine, 1 mM), exogenous nitric oxide and electrical field stimulation caused an accumulation of cGMP-like immunoreactivity in several cell-types including colonic smooth muscle cells. cGMP-like immunoreactivity was also observed in a subpopulation of neurons in both myenteric and submucosal ganglia. Sequential labeling with the NADPH diaphorase technique showed that 94% of neurons that responded to exogenous nitric oxide with an increase in cGMP-like immunoreactivity were NADPH diaphorase negative. None of the myenteric neurons that responded to electrical field stimulation with an increase in cGMP-like immunoreactivity were NADPH diaphorase positive, and only one submucosal neuron with cGMP-like immunoreactivity was also NADPH diaphorase positive. The electrical field-stimulated increase in cGMP-like immunoreactivity was blocked by nitroarginine (100 microM). An increase in cGMP-like immunoreactivity also occurred in interstitial cells located at the submucosal surface of the circular muscle layer. These cells are interposed between nerve varicosities and smooth muscle cells and may partially mediate neuromuscular transmission. Sodium nitroprusside and nitric oxide also caused an accumulation of cGMP-like immunoreactivity in smooth muscle cells of intramural arterioles and venules. The results of this study further support the role of nitric oxide as a neurotransmitter in colonic muscles, and provide support for the hypothesis that interstitial cells are functionally innervated by enteric inhibitory neurons. The data also suggest that nitric oxide may serve as a neurotransmitter in enteric ganglia.

Identification of interstitial cells in canine proximal colon using NADH diaphorase histochemistry

In gastrointestinal muscles special cells, referred to an interstitial cells, may be involved in pacemaking and transduction of inputs from the enteric nervous system. We have used a modification of the NADH diaphorase method to characterize the distribution of interstitial cells in the muscularis externa of the canine colon. The staining product of the NADH diaphorase reaction is useful because it allows light and electron microscopic studies to be performed with the same marker. Therefore rigorous identification of the cells observed at the light microscopic level could be made by electron microscopy. We were able to label at least three classes of interstitial cells: (1) at the submucosal surface of the circular muscle layer; (2) within the thickness of the circular and longitudinal muscle layers; and (3) in the region of the myenteric plexus. This technique also labeled cell bodies and initial segments of processes of Dogiel type II neurones in enteric ganglia. Nerve fibres within the muscle layers did not exhibit NADH diaphorase activity. This study has identified the interstitial cells within the circular and longitudinal muscle layers and shows the arrangement of these cells in a three-dimensional network.

Beta-adrenergic inhibition of electrical and mechanical activity in canine colon: role of cAMP

The effects of beta-adrenergic receptor stimulation on the electrical and mechanical activity of canine colonic circular muscles were compared with forskolin (Fsk), a known stimulator of adenylate cyclase. The actions of isoproterenol (Iso) were mediated by beta 2-receptors. Iso and Fsk increased intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in both the presence and absence of acetylcholine (ACh), whereas ACh (0.3 microM) alone reduced cAMP levels. These agents caused inhibition of spontaneous and ACh-induced contractions. Inhibition was associated with a reduction in the amplitude and duration of electrical slow waves recorded near the submucosal border. Near the myenteric border, Iso and Fsk hyperpolarized the membrane by up to 30 mV and changed the pattern of electrical rhythmicity. These effects were mimicked by 8-bromo-cAMP (1-3 mM). Contractile inhibition with Fsk and Iso was associated with a decrease in the amplitude and duration of Ca2+ transients measured with fura-2 fluorescence. cAMP (10-300 microM) reduced the Ca2+ sensitivity of the contractile apparatus in muscles permeabilized with staphylococcal alpha-toxin. The actions of Iso appear linked to cAMP. We hypothesize that cAMP produces relaxation both by modulation of membrane ionic channels with a consequent decline in the entry of Ca2+ as well as through a decrease in the sensitivity of the contractile apparatus to Ca2+.

Inhibition of slow-wave repolarization and Ca(2+)-activated K+ channels by quaternary ammonium ions

We studied the effects of the K+ channel blocker tetrapentylammonium (TPeA) on the electrical activity of intact circular smooth muscle from canine colon. TPeA (10 and 20 microM) increased slow-wave duration and "locked" the membrane potential around -30 mV plateau potential after several minutes of application, suggesting that K+ channels are essential for termination of colonic slow waves. Repolarization and normal slow-wave activity resumed after 20-30 min of washout. The patch-clamp technique was used to study the block of large-conductance Ca(2+)-activated K+ channels (BK channels) by TPeA and tetraethylammonium (TEA) in excised and cell-attached patches from isolated colonic smooth muscle cells. Channel block was characterized by a voltage-dependent dissociation constant [Kd(V)] for the binding of TEA and TPeA to a blocking site located a fraction of the distance across the membrane field (delta). The extracellular TEA binding site had a Kd(0) of 0.33 mM and a delta of 0.23. The extracellular TPeA binding site had a Kd(0) of 2.2 mM but showed significantly less voltage dependence (delta = 0.02). The intracellular binding site for TEA was of low affinity [Kd(0) = 76 mM]. Intracellular TPeA was the most potent blocker of BK channel current [Kd(0) = 11.7 microM]. The voltage dependence of block by intracellular TPeA (delta = -0.21) was not significantly different from that of intracellular TEA (delta = -0.3). Internal TPeA (10 microM) also blocked a 70-pS K+ channel and a 23-pS K+ channel.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Dependence of electrical slow waves of canine colonic smooth muscle on calcium gradient

1. The ionic dependence of the upstroke and plateau components of slow waves of canine colonic circular muscles was studied. 2. Reduced extracellular Ca2+ caused a decrease in the amplitude of the upstroke and plateau components, a decrease in the depolarization velocity, and a decrease in frequency. The reduction in the upstroke phase per 10-fold reduction in external Ca2+ was close to the value predicted by the Nernst relationship, suggesting that the membrane permeability to Ca2+ increases steeply during this phase. 3. Nifedipine (10(-9)-10(-6)) reduced the plateau component, but concentrations of 10(-6) M did not abolish the upstroke component. The data suggest that a nifedipine-resistant component of Ca2+ current may be involved in the upstroke. 4. Inorganic Ca2+ channel blockers (Mn2+ and Ni2+) blocked spontaneous slow waves at concentrations of 1.0 mM or less. 5. The upstroke component was more sensitive to Ni2+ than to Mn2+; a concentration of 0.040 mM-Ni2+ caused more than a 50% reduction in upstroke velocity. Ni2+ also reduced the plateau phase of slow waves. 6. The results suggest that the upstroke and plateau components of slow waves are dependent upon activation of voltage-dependent Ca2+ currents. The current responsible for the upstroke is partially resistant to dihydropyridines (at least at 10(-6) M). The current responsible for the plateau component is nifedipine-sensitive.

Upstroke component of electrical slow waves in canine colonic smooth muscle due to nifedipine-resistant calcium current

1. Electrical slow waves of gastrointestinal smooth muscles are not abolished by organic Ca2+ channel blocking drugs, such as nifedipine or D600. These compounds reduce the amplitude and duration of the plateau phase, but the upstroke phase of slow waves persists. 2. Voltage clamp experiments were performed on isolated circular muscle cells from the canine proximal colon to characterize the dihydropyridine-resistant component of inward current. Inward currents were measured at 25 and 35 degrees C. The higher temperature increased the amplitudes of the transient and sustained phases of the inward current. The voltage dependence of activation and inactivation of the inward current was not significantly changed at 35 vs. 25 degrees C. 3. At 35 degrees C the transient phase of the inward current was reduced but not blocked by nifedipine (10(-6) M). The sustained phase was blocked by nifedipine. 4. The block by nifedipine was voltage dependent, increasing with depolarization. At voltages reached during the upstroke depolarization about 35% of the inward current persisted in the presence of nifedipine (10(-6) M). This may be sufficient inward current to sustain the upstroke depolarization in intact muscles. 5. Nifedipine caused a 20 mV negative shift in the voltage dependence of inactivation suggesting that dihydropyridines may preferentially bind to Ca2+ channels in an inactivated state. 6. Ni2+ (< 100 microM) significantly decreased the transient phase of inward current. A combination of Ni2+ (40 microM) and nifedipine (10(-6) M) blocked all of the inward current at 35 degrees C. Combination of nifedipine (10(-6) M) and Ni2+ (40 microM) blocked slow waves in intact muscles. 7. Bay K 8644 (10(-6) M) increased the amplitude of the transient and sustained components of inward current. On a percentage basis the increase in the sustained component was greater than the increase in the transient component with test potentials in the range of -50 to -20 mV. This may explain why Bay K 8644 preferentially increases the plateau component of slow waves vs. the upstroke component. 8. The findings of this study suggest that the nifedipine resistance of the upstroke depolarization could be due to the voltage dependence of the block of Ca2+ channels by dihydropyridines. Thus a single class of voltage-dependent Ca2+ channels could be responsible for the upstroke and plateau phases of slow waves.

NADPH diaphorase and nitric oxide synthase colocalization in enteric neurons of canine proximal colon

Considerable evidence has recently been presented that suggests that nitric oxide (NO) is a nonadrenergic noncholinergic (NANC) neurotransmitter in gastrointestinal tissues. One of the criteria that must be satisfied before this hypothesis can be accepted is that enteric neurons must be shown to contain the enzymatic apparatus necessary to synthesize NO. Specific antibodies have been developed for NO synthase (NOS) isolated from rat cerebellum, and studies have shown that NOS copurifies and colocalizes with NADPH diaphorase activity, a commonly used neural marker. We used antibodies raised against the cerebellar NOS to determine the distribution of NOS-like immunoreactivity (NOS-LI) in enteric neurons of the canine proximal colon. We also tested whether NADPH diaphorase staining would label the population of neurons containing NOS-LI in this species. A subpopulation of neurons in myenteric and submucosal ganglia displayed NOS-LI and were colabeled with NADPH diaphorase. Labeled neurons had morphological characteristics similar to the Dogiel type I morphology. Cryostat sections showed NOS-positive nerve trunks throughout the circular and longitudinal muscle layers, but a high density of NOS-LI was observed within the submucosal pacemaker region, as predicted from physiological studies. These studies provide the first morphological support for the hypothesis that NO serves as a NANC neurotransmitter in the canine colon. The study also shows that the NADPH diaphorase reaction provides a useful method to label cells with NOS-LI.

Participation of fast-activating, voltage-dependent K currents in electrical slow waves of colonic circular muscle

The plateau phase of electrical slow waves in phasic gastrointestinal muscles is critical for excitation-contraction coupling. The plateau appears to depend upon a balance between inward Ca2+ current and outward K+ currents that is sustained for several seconds. Voltage-dependent, non-Ca(2+)-dependent K currents were studied in canine colonic circular muscle cells using the whole cell patch-clamp technique. At room temperature, depolarization activated a slow outward current that showed little inactivation during 500 ms. Increasing the temperature to 37 degrees C significantly increased the rate of activation of voltage-dependent outward current. The onset of the outward current overlapped the transient inward Ca2+ current, suggesting that this K current may act as a brake on the upstroke depolarization of electrical slow waves in intact muscles. Voltage-dependent outward current was sustained for the duration of test pulses. This current balanced the sustained inward current that was also activated at physiological test potentials. The outward current evoked by test pulses positive to -20 mV inactivated by at least 50% within 500 ms. Half inactivation occurred at -36 mV. Voltage-dependent K current was reduced by 4-aminopyridine (4-AP; 1-5 mM), but difference currents obtained by subtracting currents elicited from holding potentials of -45 mV from currents obtained from holding potentials of -100 mV were not affected by 4-AP (1 mM). Studies were also performed on intact muscles to test the effects of 4-AP on electrical slow waves. 4-AP increased the amplitude and rate of rise of the upstroke potential and increased the amplitude and prolonged the plateau phase of slow waves. These data suggest that a rapidly activating, inactivating, voltage-dependent K current participates in electrical slow waves of colonic circular smooth muscles.

Outward currents in longitudinal colonic muscle cells contribute to spiking electrical behavior

Electrical events in longitudinal and circular muscles of the colon are different. Longitudinal muscles generate action potentials superimposed upon small depolarizations termed myenteric potential oscillations and circular muscles generate slow wave events that persist for several seconds. Differences between circular and longitudinal muscles may be related to the potassium channels these cells express. We have studied Ca(2+)-dependent and voltage-dependent K currents of isolated longitudinal cells with the whole cell patch-clamp technique. Test depolarizations positive to -40 mV yielded a transient inward current followed by a large sustained outward current. Blockade of the inward Ca2+ current reduced the amplitude of the outward current. Outward current was also reduced by tetraethylammonium (TEA; 1 mM), suggesting that a component of the outward current is Ca2+ dependent. After blockade of the Ca(2+)-dependent outward current, a voltage- and time-dependent component of outward current remained. The activation and inactivation properties and sensitivity to TEA and 4-aminopyridine (4-AP) were characterized. The voltage-dependent outward current in longitudinal cells had different properties than the voltage-dependent K currents in circular muscle cells (i.e., more negative inactivation, less sensitivity to 4-AP). TEA (1-5 mM) increased the amplitude and frequency of action potentials in intact longitudinal muscles; 4-AP (1 mM) had little effect on electrical activity of longitudinal muscles. The data suggest that differences in electrical behavior of the 2 muscle layers may be related to the expression of different species of K channels.

Role of nitric oxide in non-adrenergic, non-cholinergic inhibitory junction potentials in canine ileocolonic sphincter

1. Electrical field stimulation causes neurally-mediated relaxation of the ileocolonic sphincter that is due to activation of non-adrenergic and non-cholinergic (NANC) nerves. Recent studies have suggested that nitric oxide (NO) is the neurotransmitter that mediates relaxation. 2. Using intracellular recording techniques, we have tested whether NANC inhibitory junction potentials (i.j.ps) in the canine ileocolonic sphincter are also mediated by NO. 3. Electrical field stimulation elicited excitatory and inhibitory junction potentials: e.j.ps were blocked by atropine (10(-6) M) and tetrodotoxin (TTX; 10(-6) M); i.j.ps were also blocked by TTX and partially blocked by apamin (10(-6) M). I.j.ps were unaffected by atropine, phentolamine and propranolol (all at 10(-6) M). 4. The arginine analogues, L-NG-nitroarginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), decreased the amplitude of i.j.ps and L-arginine, but not D-arginine, partially restored the i.j.ps. 5. I.j.ps were also inhibited by oxyhaemoglobin (1%), but not by methaemoglobin. 6. Exogenous NO (10(-7) M to 3 x 10(-5) M) caused concentration-dependent hyperpolarizations that were similar in amplitude to the NANC nerve-evoked i.j.ps. Hyperpolarizations to NO were unaffected by L-NAME, but were blocked by oxyhaemoglobin. 7. Tetrodotoxin, L-NAME and oxyhaemoglobin all caused depolarization of resting membrane potential. 8. The specific guanosine 3':5'-cyclic monophosphate phosphodiesterase inhibitor, M&B 22948, caused hyperpolarization, increased the maximum level of hyperpolarization reached during i.j.ps, and increased the duration of i.j.ps. 9. These data further support the hypothesis that NANC neurotransmission in the ileocolonic sphincter is mediated by NO or an NO-releasing compound. The data also suggest that tonic release of NO, possibly from spontaneous firing of NANC nerves, may regulate resting membrane potential and tone in this sphincter.

Nitric oxide as a mediator of nonadrenergic noncholinergic neurotransmission

Part of the regulation of gastrointestinal (GI) smooth muscles is provided by nonadrenergic noncholinergic (NANC) nerves. Stimulation of these nerves, either by field stimulation or via neural reflex pathways, elicits hyperpolarization of postjunctional smooth muscle membranes referred to as inhibitory junction potentials and relaxation. The transmitter(s) that mediate NANC inhibitory neural transmission have been a controversial topic for nearly 30 years. Recent evidence suggests that nitric oxide (NO) may serve as a NANC inhibitory transmitter in the GI tract. This hypothesis is supported by the following. 1) Immunohistochemical studies have shown that the enzyme necessary for NO synthesis is expressed in enteric neurons. In vitro studies of muscles from nearly all levels of GI tract have also shown that arginine analogues, which inhibit NO synthesis, reduce inhibitory effects of NANC neurotransmission. Effects of arginine analogues can be restored by addition of excess L-arginine, the substrate for NO synthesis. These data suggest that NO can be synthesized by enteric nerves. 2) Bioassays have demonstrated nerve-evoked release of a substance that has been identified as NO during NANC nerve stimulation. Oxyhemoglobin, known to bind to and sequester NO, also blocks NANC responses. These data suggest that NO is released into extracellular fluid during nerve stimulation. 3) Addition of NO causes rapid hyperpolarization of GI smooth muscle cells and relaxes muscles strips. These effects are similar to NANC nerve responses. NO and electrical field stimulation also increase tissue guanosine 3',5'-cyclic monophosphate, which may be the second messenger involved in NANC responses. 4) Removal of NO is easily accomplished by its rapid spontaneous breakdown in physiological solutions. 5) The pharmacology of NO and the NANC neurotransmitter in many preparations is similar, e.g., oxyhemoglobin blocks responses to NANC nerve stimulation and to exogenous NO. In summary, it would appear that many of the criteria necessary for NO to be considered a neurotransmitter have been satisfied.

Cigarette smoking and outcomes of in-vitro fertilization and embryo transfer: a prospective cohort study

A prospective cohort study of 222 consecutive couples undergoing 297 cycles of in-vitro fertilization and embryo transfer (IVF-ET) was conducted to evaluate the impact of cigarette smoking in males and females. Compared with non-smokers, females smoking at the time of treatment had more previous pregnancies (1.16 versus 0.63, P less than 0.001), consumed more coffee per day (3.29 versus 1.85 cups, P = 0.001) and were less likely to hold a professional or skilled job (41% versus 66%). There was no difference in the response to ovarian stimulation in terms of the duration and dose of human menopausal gonadotrophin, peak oestradiol level or number of oocytes retrieved. The fertilization rate was actually higher in heavy smokers than in non-smokers (79.3% versus 61.3%, P = 0.007). The rate of embryo cleavage was retarded in a dose-dependent fashion. In smokers of 1-14 cigarettes/day, the likelihood of transferring an embryo at greater than or equal to 4-cell stage was 0.87 [95% confidence limits (CL) 0.56-1.4] and in smokers of greater than or equal to 15 cigarettes/day, the likelihood was 0.52 (95% CL 0.31-0.88). However, evaluation of interrelated factors using logistic regression suggested that a low socioeconomic status had a greater detrimental effect on embryo cleavage rate than female smoking. No significant difference was noted in the clinical outcome following embryo transfer. A study of larger sample size is required to evaluate whether the effects of cigarette smoking are independent of socioeconomic status and other related factors and whether they result in reduced ongoing clinical pregnancy and live birth rates.

Electrical and mechanical effects of acetylcholine and substance P in subregions of canine colon

Effects of acetylcholine (ACh) and substance P on the electrical and mechanical activities of the circular muscle layer of the canine proximal colon were studied. Because this muscle layer is bordered by two different pacemaker regions, responses from segments containing either a single pacemaker region or no pacemaker region were compared with responses of the complete muscle layer. Concentration-response relationships for ACh and substance P were similar between the various segments, suggesting that receptors for these agonists are expressed throughout the layer. The dominant contractile pattern induced by ACh and substance P in each segment was a 1- to 3-cycle/min rhythm. In a like manner, these agonists also elicited an electrical pattern in which a long-duration slow wave occurred one to three times per minute between short-duration slow waves. Low concentrations of nifedipine (0.01 microM) selectively antagonized the 1- to 3-cycle/min rhythm. In circular muscles with no pacemaker region, ACh (1 microM) caused depolarization, induced oscillations in membrane potential averaging 24 +/- 5 mV in amplitude and 2.9 +/- 0.9 cycles/min in frequency, and generated rhythmic contractions at the same frequency. This "interior" circular muscle was functionally innervated by cholinergic excitatory nerves. Exposure to ACh (1 microM) did not alter the conduction of slow waves through the thickness of the circular layer. In summary, the excitatory neurotransmitters, ACh and substance P, induce a dominant electrical and contractile rhythm throughout the circular muscle layer that is different from the spontaneous rhythms produced at either the myenteric or submucosal border.

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.

Use of adaptive behavior measures and overrepresentation of black students in programs for students with mild mental retardation

The use of adaptive behavior information for classification and placement of black and white students with mild mental retardation was compared and overrepresentation of black students in special classes was analyzed. Adaptive behavior measures were not used to the same extent as general intelligence tests for either group, despite court decisions and recommendations by authoritative sources. Direct comparisons on general intellectual functioning and several indices of adaptive behavior yielded few differences, suggesting that an equal treatment conception of fairness was achieved despite substantial overrepresentation of black students in programs for students with mild mental retardation. Use of adaptive behavior needs to improve significantly to comply with legal requirements.

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.

Inhibition of electrical slow waves and Ca2+ currents of gastric and colonic smooth muscle by phosphatase inhibitors

The effects of calyculin A, a phosphatase inhibitor isolated from the marine sponge Discodermia calyx, on the electrical activity of colonic and gastric muscles were studied. Calyculin A reduced the amplitude and duration of slow waves, primarily by inhibiting the plateau component. Okadaic acid, another phosphatase inhibitor, also reduced the amplitude and duration of gastric slow waves. The mechanism of action of calyculin A was investigated by studying its effects on inward currents of isolated gastric and colonic myocytes. Calyculin A reduced the amplitude of the peak and the sustained components of the inward current. Okadaic acid had similar effects. These data suggest that phosphorylation of Ca2+ channels of gastrointestinal smooth muscles may inhibit Ca2+ currents. This mechanism may provide an important means of regulating the currents responsible for excitation-contraction coupling in these muscles.

Structure and organization of electrical activity of canine distal colon

The morphology and electrophysiology of the canine distal colon were studied to compare this region with the proximal colon. Many morphological characteristics were similar including the presence of interstitial cells at the submucosal surface of the circular layer. Muscle cells near the submucosal surface had resting membrane potentials (RMPs) of -79 +/- 1 mV, and slow waves were generated in this region. Slow waves had similar waveform characteristics to those of the proximal colon, but rapid oscillations were superimposed on slow waves of some preparations. RMPs and slow waves decreased with distance from the submucosal surface. The latter were not resolvable in the myenteric half of the circular layer. Cells at the myenteric border had RMPs of -49.5 +/- 2 mV and a higher frequency oscillation of 16 min-1. Acetylcholine increased slow-wave amplitude and duration and caused fast oscillations on the plateau phase of slow waves. Isolated circular myocytes were studied with the patch-clamp technique. Cells from the submucosal border displayed voltage-dependent inward and outward currents. With outward currents blocked, the inward current was composed of two components. Nifedipine (10(-6) M) blocked a portion of the inward current but left a substantial transient component. The effect of nifedipine correlated with its effects on tissues, suggesting that two components of Ca2+ current participate in slow waves. These studies describe numerous similarities in the structure and activity of the proximal and distal portions of the colon but also show some potentially important differences between these regions.

Involvement of nitric oxide synthetic pathway in inhibitory junction potentials in canine proximal colon

Experiments were performed to determine the involvement of nitric oxide synthase in the generation of nonadrenergic, noncholinergic (NANC) inhibitory junction potentials (IJPs) in the canine proximal colon. Smooth muscle cells were impaled near the myenteric border between the circular and longitudinal layers. Cells exhibited rhythmic myenteric potential oscillations. IJPs were evoked with electrical field stimulation in the presence of drugs to block adrenergic and cholinergic neurotransmission. N omega-nitro-L-arginine, methyl ester (L-NAME), a nitric oxide synthase inhibitor, progressively reduced the amplitude of IJPs. The effect of L-NAME was reversed by L-Arg but not by the stereoisomer D-Arg. IJPs disrupt the regular pattern of myenteric potential oscillations. This effect was also blocked by L-NAME and reversed by L-Arg. These experiments suggest that a product of the nitric oxide synthetic pathway is involved in NANC neurotransmission in the canine proximal colon.

Correlation between electrical and morphological properties of canine pyloric circular muscle

Electrical slow waves decay in amplitude as they conduct from the myenteric to the submucosal regions of the circular muscle layer in the canine pyloric sphincter. We used the partitioned chamber method to study the passive and active properties of pyloric muscles, and we found that length constants of circular muscles of myenteric region were significantly longer than muscles near the submucosal surface. These data suggested differences in either membrane resistance, junctional resistance, or cytoplasmic resistance. The first parameter was evaluated by measuring time constants in intact tissues and single cells isolated from the submucosal and myenteric regions. Membrane time constants were not different in the two regions, nor were differences found in the input resistances of isolated cells. Morphological studies failed to demonstrate differences in cell diameters in the two regions suggesting that cytoplasmic resistances are similar. These findings suggest that the different cable properties in the two regions may be due to differences in electrical coupling. Morphological examination revealed similar numbers of gap junctions between cells in the two regions, but large differences were noted in the size of muscular bundles. Muscles of the myenteric region were arranged into large, tightly packed bundles, whereas muscles of the submucosal region consisted of small bundles with an extensive extracellular space filled with connective tissue. We suggest that the difference in cable properties may be due to differences in electrical coupling between bundles. These data suggest that submucosal muscles function more like a multiunit smooth muscle, whereas myenteric muscles behave as a single unit.

Comparison of the hematologic effects of 2-butoxyethanol using two types of hematology analyzers

Earlier reports from this laboratory indicated that 2-butoxyethanol (BE) causes acute hemolytic anemia in rats as evidenced by a time- and dose-dependent decrease in the number of red blood cells, in hemoglobin concentrations, and in hematocrits (HCT). Subsequent studies showed that treatment with BE causes an early increase in HCT and mean cell volume (MCV). Since this effect went undetected in our early work and resulted in the publication of inaccurate information, present studies were designed to reinvestigate the hematologic effects of BE using the laser-based hematology analyzer which was used in the early studies and an impedance-based hematology analyzer, simultaneously. Packed cell volumes (PCV; spun HCT) were also performed on all blood samples. Male F344 rats were treated with 0, 125, 250, or 500 mg BE/kg (po) and blood was collected from the retro-orbital venous plexus at 1, 2, 4, 8, and 24 hr after dosing. Hematology profiles of BE-treated rats obtained from the impedance-based analyzer showed an early dose- and time-dependent increase in HCTs and MCVs. In contrast, analysis of the same blood samples using the laser-based analyzer showed a dose- and time-dependent decrease in HCTs with little or no change in MCVs. Changes observed in PCVs were consistent with results obtained from the impedance-based analyzer. Therefore, under the experimental conditions of this and previous studies, the laser-based analyzer was unable to detect early increases in HCTs and MCVs in rats treated with BE. Finally, these data explain the different principles utilized to measure cell size by both instruments as they relate to the hematologic effects of BE. Moreover, present data show that BE-induced hemolysis of erythrocytes is preceded by a quantitatively unique and massive swelling suggesting the erythrocyte membrane as the target.

Use of rhodamine 123 to label and lesion interstitial cells of Cajal in canine colonic circular muscle

The role of interstitial cells of Cajal (ICC) is difficult to determine because these cells are not easily identified by light microscopy, and there are no compounds available to specifically lesion ICC. Ultrastructural studies have shown an abundance of mitochondria in ICC. Therefore, we have used rhodamine 123, a fluorescent dye that is specifically accumulated by mitochondria, to identify ICC in canine proximal colon. This technique provided good discrimination between ICC and smooth muscle cells, but enteric neurons were labeled with rhodamine 123. This compound has cytotoxic properties in some cells. Therefore, we treated intact muscle strips with rhodamine 123 while recording intracellular electrical activity from circular muscle cells. Uptake of rhodamine 123 by ICC was associated with an alteration in electrical rhythmicity. These data suggest that rhodamine 123 may be a useful tool for visualizing and perhaps chemically lesioning ICC.

Pacemaker activity in septal structures of canine colonic circular muscle

Morphological and electrophysiological experiments were performed to characterize the pacemaker areas of the circular muscle in the canine proximal colon. Morphological studies showed interstitial cells of Cajal lining the submucosal surface of the circular layer and the septal structures that separate the circular layer into bundles. Electrical measurements suggested that slow waves may propagate into the thickness of the circular muscle in a regenerative manner along the surface of these septa. Removal of the submucosal pacemaker region blocked generation of slow waves in nonseptal regions of the circular muscle, but slow-wave activity continued in the circular muscle near septa. These data suggest that slow-wave pacemaker activity is not limited to a two-dimensional surface at the submucosal surface but extends into the interior of the circular layer along septal invaginations. Experiments were also performed to determine the dominance of pacemaker activity (i.e., septal vs. submucosal), and examples were found in which both areas appeared to initiate slow waves in intact muscles. Other studies showed that slow waves could propagate across septa, suggesting some form of electrical coupling between circular muscle bundles. This study provides a more complete view of the structure and function of pacemaker areas in the canine proximal colon.

Spontaneous electrical activity of interstitial cells of Cajal isolated from canine proximal colon

Interstitial cells of Cajal (ICC) have been suggested as pacemaker cells in the gastrointestinal tract. A method was developed to isolate ICC from the slow-wave pacemaker region of the canine proximal colon. These cells were identified under phase-contrast microscopy, and their identity was verified by comparing their ultrastructure with the morphology of ICC in situ. Patch-clamp experiments demonstrated that these cells are excitable; voltage-dependent inward and outward currents were elicited by depolarization. Inward current transients were identified as calcium currents. A portion of the outward current appears to be due to Ca2+-activated K channels commonly expressed in these cells. ICC were also spontaneously active, generating electrical depolarizations similar in waveform to slow-wave events of intact colonic muscles. These findings are consistent with the hypothesis that ICC initiate rhythmicity in the colon.

Dexamethasone effects on ritodrine-induced changes in myometrial contractility and beta-adrenergic receptor function

We have previously demonstrated in pregnant sheep that ritodrine infusion for 24 hours reduces myometrial beta-adrenergic receptor density and isoproterenol-stimulated adenylate cyclase activity. These receptor-associated changes were accompanied by an increasing inability of ritodrine to inhibit uterine contractility induced by a bolus of oxytocin. In the present study, we evaluated whether these ritodrine-induced effects could be altered by dexamethasone. Ten pregnant sheep at gestational ages of 92 to 130 days received ritodrine 2 micrograms/kg/min for 24 hours. Five animals also received dexamethasone 10 mg intravascularly twice during the ritodrine infusion. Before and at 4 and 24 hours of ritodrine infusion, the animals were given an identical dose of oxytocin as a bolus, and the area under the uterine pressure-time curve was quantified. Myometrial biopsy specimens were obtained before and after ritodrine infusion. Dexamethasone treatment prevented ritodrine-induced reductions in beta-adrenergic receptor density and isoproterenol-stimulated adenylate cyclase activity. Despite these receptor-associated effects, dexamethasone did not prevent the loss of tocolytic efficacy associated with prolonged ritodrine infusion.

Myometrial desensitization after ritodrine infusion

We have developed a model in pregnant sheep to investigate pharmacological agents used for suppression of preterm labor. This model allows repetitive determinations of uterine contractility and simultaneous measurements of myometrial receptor and postreceptor events. We used the model to study the beta-adrenergic agent ritodrine. We infused 11 pregnant sheep with ritodrine and 3 with physiological saline for 24 h. Oxytocin boluses were given before and at 5 and 22 h after onset of the infusion. Myometrial biopsies were obtained before and immediately after the infusion. After 22 h of ritodrine infusion, uterine contractility in response to the same oxytocin bolus was 50% greater than at 5 h (P less than 0.02). Myometrial membrane beta-adrenergic receptor density decreased 49% (P less than 0.005), and catecholamine-stimulated adenylate cyclase activity was reduced 70% (P less than 0.005). The model thus demonstrates that use of the beta-adrenergic agonist ritodrine in a clinically relevant manner results in tachyphylaxis of its effects on both physiological parameters and the receptor cascade system.

Structure and ultrastructure of muscle systems within Grillotia erinaceus metacestodes (Cestoda: Trypanorhyncha)

Three distinct muscle types have been identified within the metacestode of Grillotia erinaceus. These consist of peripheral somatic myofibres plus two muscle systems directly involved in parasite attachment to the host, i.e. the tentacular bulb and its antagonistic retractor muscle. In common with other cestodes the somatic muscle consists of smooth-type fibres running longitudinally and obliquely to the main body axis. The retractor muscle consists of myofibres with centrally displaced nuclei. Upon contraction these latter fibres become spirally orientated causing the muscle to coil and lateral membranes to become elevated as spikes. Definitive nerve processes have not been identified within somatic or retractor muscle. Individual tentacular bulbs form the proximal terminus for a closed hydraulic system. Each bulb consists of overlapping, contrarotating myofibres which display obvious striations; the striations appear in alternate fibres to be in transverse and oblique planes. Adjacent myofibres are separated by approximately 0.5 micron, possess abundant mitochondria and have shallow t-tubules plus associated vesicles of sarcoplasmic reticulum at each Z line. Thick myofilaments are surrounded by 13, shared, thin myofilaments. Close neuronal control for the bulb muscle is suggested by the presence of obvious motor end-plates which contain both lucent and dense neurovesicles.

Polyhydramnios and obstructive renal failure: a case report and review of the literature

Described is a pregnancy complicated by pregnancy-induced hypertension, polyhydramnios, and obstructive renal failure due to an overdistended uterus. A review of the literature disclosed that only five such cases have been reported previously. Fetal outcome was generally related to the duration of gestation at the onset of polyhydramnios.

The role of cartilage canals in the formation of secondary centres of ossification

The present studies indicate that in the growth of cartilage canals the cartilage is removed by chondroclasts which stem from the primitive connective tissue cells of the perichondrium. Besides fusing to form multinucleated chondroclasts, these cells also provide the osteoblasts which establish the secondary centre of ossification. The growing tips of the blood vessels within the canals are also fashioned from these primitive connective tissue cells as they invade the microhaemorrhages at the ends of the canals. This is an identical procedure to that found in the vascular invasion of the primary growth plate. The cartilage canals are clearly useful sites in which to study the development of osteoclasts and chondroclasts from more primitive (osteoprogenitor) cells.

Microcytosis, anisocytosis and the red cell indices in iron deficiency

Red cell volume distribution curves have been used to measure microcytosis and anisocytosis in normal subjects, blood donors and patients with iron deficiency anaemia. These measurements were more sensitive than the conventional red cell indices for detecting blood donors with a low transferrin saturation. Three stages are suggested as iron deficiency progressively interferes with haemopoietic function. Anisocytosis and an increased percentage of microcytic cells are the first haematological abnormalities to occur and at this stage haemoglobin concentration is usually normal and trasferrin saturation less than 32%. At the second stage the MCV and MCH decline, haemoglobin concentration is generally sub-normal, though not below 9 g/dl, and transferrin saturation is usually below 16%. The final stage of iron deficiency is associated with a low MCHC, a haemoglobin concentration below 9 g/dl and a transferrin saturation of less than 16%.