Mechanism of neurotensin depolarization of rabbit colonic smooth muscle

Modulation of electrical activity in gastrointestinal smooth muscle by peptides

A rise in intracellular calcium is the predominant signal that leads to the activation of the contractile machinery in gastrointestinal smooth muscle. The primary sources of activating calcium are illustrated in Fig. 2. Voltage- and peptide-mediated release of intracellular calcium contribute to activation of some gastrointestinal smooth muscles. However, the primary source of activating calcium appears to be an influx of calcium across the plasma membrane. The degree of modulation of electrical activity by peptides varies depending upon the region of the gastrointestinal tract studied. Second messenger systems are undoubtly involved in the transduction pathway for receptor-mediated changes in ion channel activity in gastrointestinal smooth muscle. However, in comparison to other excitable cell types, little is known about the coupling mechanisms whereby peptide-receptor binding alters ion channel activity in gastrointestinal smooth muscle. This represents one of the challenging areas to be studied in the field of gastrointestinal smooth muscle. One disease in which a better appreciation of the regulation of ion channel activity could lead to therapeutic benefit is irritable bowel syndrome. A coupling of smooth muscle electrical activity to hypermotility in irritable bowel syndrome has been reported. CCK increases the level of spike activity which triggers hypermotility [40]. It would follow that inhibition of calcium influx should reduce spiking and, therefore, hypermotility. In fact, the calcium channel blockers nifedipine and nicardipine have been shown to decrease colonic motility in irritable bowel syndrome patients [62-64]. As our understanding of gastrointestinal smooth muscle ion channels expands, development of a gastrointestinal selective calcium channel blocker may be possible. This class of agents would be effective in the treatment of irritable bowel syndrome and potentially other peptide-related spastic smooth muscle disorders.

Effect of neurotensin on contractile activity and [3H]acetylcholine release in cat terminal ileum during different postnatal periods

The effect of neurotensin (NT) on the contractile activity of circular and longitudinal strips from the terminal ileum of 15-, 30-, 60-day-old and adult cats as well as on the resting and electrically-evoked release of [3H]acetylcholine (ACh) was studied. Radioactivity was measured by liquid scintillation spectrometry and the effect of NT was evaluated by the S2/S1 ratio. In the circular muscle strips NT (1-100 nM) inhibited spontaneous contractions in all age groups. In the longitudinal strips the effect of NT was concentration- and age-dependent. NT at a concentration of 1 nM had no effect on the spontaneous activity in 15-day-old cats, but in the other age groups in 70-80% of the cats it inhibited spontaneous contractions. The response to 10 and 100 nM NT was either biphasic (relaxation followed by contraction) or inhibitory: in 15-day-old cats the response was biphasic only and with increasing age the percentage of strips responding with inhibition of the contractions increased. Neither substances affecting adrenergic and cholinergic transmission nor TTX changed the inhibitory response to NT. The contractile component of the biphasic response was TTX-resistant in all age groups and was significantly decreased by scopolamine in 60-day-old and adult cats. NT increased both resting and electrically-evoked release of [3H]ACh which was not changed by TTX. In the presence of the peptide the S2/S1 ratio increased as NT-induced [3H]ACh release in the strips of adult cats was higher than that in young cats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the response of proximal and distal rabbit colonic muscle after electrical field stimulation

Electrical field stimulation (EFS) was performed on rabbit proximal and distal circular colonic smooth muscle to study the mechanisms of neural control of the colon. Electrical pulses were applied with parallel silver plate electrodes to muscle that had been stretched to Lo. The proximal muscle demonstrated an on-contraction during EFS. In distal muscle, EFS initiated an on-relaxation, followed by an on-contraction and an off-contraction. The time delay for the on-contraction of distal muscle was longer by 2.5 +/- 0.5 s than was the delay in proximal muscle (p less than 0.02). The amplitudes of the on- and off-contraction were dependent on the frequency of the EFS. The on- and off-responses were completely inhibited by 3 x 10(-6) M tetrodotoxin. Atropine inhibited the distal on-contraction at all EFS frequencies and the proximal on-response at EFS frequencies less than 16 Hz. Atropine had a partial inhibitory effect on the distal off-response (approximately 30%). Bombesin and substance P were released during prolonged EFS. Desensitization of the distal colonic muscle to bombesin did not affect the distal off-contraction. However, desensitization of the tissue to substance P and exposure to substance P antagonists inhibited the distal off-contraction. These studies suggest that (a) acetylcholine mediates the on-contraction of the distal circular colonic muscle, and a major part of the on-contraction for the proximal muscle, and (b) substance P is responsible for the off-contraction of the distal muscle.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Calcium dependence of neurotensin stimulation of circular colonic muscle of the rabbit

The effect of neurotensin on smooth muscle contraction was compared in strips from rabbit proximal and distal circular colonic muscle. The effective dose for neurotensin stimulation that caused a 50% response in both tissues was similar (1.3 X 10(-10) M). The maximal isometric stress, however, was greater in the distal colon than in the proximal colon (p less than 0.01). Neurotensin stimulation of both proximal and distal colon was unaffected by tetrodotoxin, phentolamine, propranolol, naloxone, or atropine. Neurotensin-stimulated contraction was inhibited by "Ca2+-free" (pCa = 5.1) or La3+ buffer. Verapamil (10(-6) M) or nitroprusside (10(-4) M) decreased neurotensin stimulation of proximal and distal colon by approximately 40% (p less than 0.05). Removal of Ca2+ from the buffer inhibited stimulation of muscle contraction by high extracellular potassium [( K+]o) more than bethanechol stimulation (p less than 0.01). La3+ (1 mM) inhibited the contraction stimulated by bethanechol or increased [K+]o. Although verapamil inhibited contraction by bethanechol and increased [K+]o by approximately 50%, nitroprusside had no effect on the contraction mediated by these stimulants. 8-Bromo-guanosine 3',5'-cyclic monophosphate (cGMP) inhibited neurotensin, but not [K+]o or bethanechol-stimulated contraction. These data suggest (a) neurotensin stimulated colonic contractions at a concentration that is potentially physiologic, (b) neurotensin stimulated colonic smooth muscle directly without neural mediation, (c) neurotensin stimulation of colonic muscle is controlled by [Ca2+]o and [cGMP]i.