Temperature-dependent interconversion of histamine H1 and H2 receptors in guinea pig ileum

Effect of cimetidine on the histamine and dimaprit induced responses on the guinea pig ileum during development

1. In ileal segments obtained from 1, 15, 30, 45 and 60-day-old guinea pigs, histamine produced a dose-dependent significant contractile response from a threshold concentration of 3.2 x 10(-8) M. 2. The mean contractile responses (at 10(-5) M) of the ileum from the 1, 15, 30 and 45-day-old guinea pigs compared to that from the 60-day-old ones were 73.79, 92.30, 94.07 and 94.41% respectively. 3. Dimaprit (at 10(-5) M) had no effect on the ileum of the 1-day-old guinea pigs, while it produced a significant contractile response on the ileum of the guinea pigs at all other ages. 4. The mean contractile responses of the ileum from the 15, 30 and 45-day-old guinea pigs compared to that from the 60-day-old ones were 74.44, 66.66 and 76.94% respectively. 5. Cimetidine (at 10(-5), 3.2 x 10(-5) and 10(-4) M) significantly potentiated the histamine-induced contraction on the ileum of the 1-day (from 10(-7) to 3.2 x 10(-6) M) and the 15-day-old guinea pigs (at 10(-8) M). 6. On the contrary, cimetidine (at 3.2 x 10(-5) M) significantly inhibited the contractile response of the ileum to dimaprit. 7. In conclusion, the guinea pig ileum appears increasingly sensitive to histamine from the newborn to the 60-day-old ones, but its sensitivity to dimaprit, not detectable in the newborn, fluctuates during development. 8. Cimetidine produces a potentiating effect, which declines with age, on the on the histamine-induced contraction, while it inhibits the dimaprit contractile response on the ileum.

Histamine paw edema of mice was increased and became H2-antagonist sensitive by co-injection of nitric oxide forming agents, but serotonin paw edema was decreased

Nitric oxide (NO) surprisingly caused the opposite effect on histamine and serotonin edema. The local injection of acidified nitrite (0.3-30 micrograms/paw which correspond 10 micrograms-1 mg/kg) increased histamine edema of mice up to 45 +/- 4% and suppressed serotonin edema to 90 +/- 3%. Other NO-generators (nitroprusside sodium and hydroxylamine) showed similar effects. These results were in accordance with our previous data on endogenous NO. Methylene blue (MB, 30 ng/paw which corresponds to 1 microgram/kg) suppressed histamine edema (62 +/- 3%) and increased serotonin edema (43 +/- 3%) in normal mice, being reversed by acidified nitrite. This suggests the involvement of guanosine 3',5'-cyclic monophosphate (cGMP) formation for the action of NO. Histamine edema became sensitive to H2-antagonist, cimetidine, by co-injection of 30 micrograms/paw (which corresponds to 1 mg/kg) acidified nitrite (ED50 = 30 micrograms/kg versus >> 1 mg/kg). NO seemed to modify the histamine receptor(s) or tautomeric form of histamine. NO, O2- and other oxyradicals might finely control the vascular permeability together with inflammatory mediators.

Changes in the ionic environment may alter the kind of antagonism of some histamine H2-receptor blockers in the guinea pig papillary muscle

The effect of changes in the composition of the bathing medium on the effect of histamine and histamine H2-receptor antagonists was investigated in the isolated guinea pig papillary muscle. Ringer or Krebs-Henseleit solutions were used as nutrient fluids. They mainly differed with respect to pH and to Mg2+ and H2 PO4- content. Whereas the effect of histamine was not altered by ionic changes, the antagonism by some H2 blockers was different in the two nutrient solutions. The insurmountable antagonism elicited by high concentrations (greater than or equal to 10(-6) M) of famotidine, oxmetidine and mifentidine in Ringer solution was converted to surmountable when these drugs were tested in Krebs-Henseleit solution. Conversely, the antagonism induced by ranitidine was surmountable in both solutions, and that induced by high amounts of Ioxtidine was insurmountable in both nutrient fluids. Results obtained in Ringer solution were not modified by pH adjustments or by the addition of ions present in Krebs-Henseleit medium. These results suggest that the interaction of histamine with H2 receptors in the guinea pig papillary muscle was not influenced by alterations in the ionic composition of the nutrient fluid, whereas the antagonism may be critically dependent on the ionic environment.

Receptors in the gastrointestinal tract

The receptor concept has been recently evolved and a new science was actually created, namely "receptorology". Receptors are now identified by means of different techniques (binding, agonist-antagonist interaction, autoradiography, etc.). The new techniques allowed the investigators to define new receptors and new subtypes of the "classical" ones. In the gastrointestinal (GI) tract a number of receptors have been identified and localized both on the effector organ and in the nerve terminal where they exert an important modulatory function on the neurotransmitter release. Recent biochemical studies have allowed a better understanding of the post-receptor event involving the second or third messenger regulation. Particular changes of receptors were recognized and they allow us to consider receptors not as static entities but as very dynamic components of the plasma membrane capable of different kinds of alterations, like interconversion, internalization, mobility, up- and downregulation, etc. Together with the "classical" receptors (cholinergic, adrenergic, opioid, etc.) also new receptors were identified: different subtypes of receptors for the tachykinins, for prostaglandin of the E type in the gastric parietal cell and the so-called dihydropyridine (DHP) receptor in the calcium channel of different areas of the gut. It is obvious that the precise knowledge of receptors and of their agonists and antagonists will represent the basis for a more specific and efficacious treatment of various gastrointestinal disorders.

Preservation of taenia coli by freezing and storage at -196 degrees C

Some damaging effects that occur during cryopreservation by freezing to -196 degrees C have been evaluated in rabbit taenia coli by analyzing the proportional recovery of acetylcholine- and histamine-induced maximal contractions. Dimethyl sulfoxide (Me2SO) 10 v/v% was used as the cryoprotectant; it reversibly abolishes spontaneous contractility even after incubation at 37 degrees C during 2 hr. Programmed freezing at 0.6 degrees C/min with compensation for the latent heat of fusion and warming at 35 degrees C/min proved to be slightly superior to programmed cooling without compensation and slower warming. The degree of functional recovery was comparable after either abrupt or stepwise removal of Me2SO. Freeze-thawing resulted in a significant reduction of contractile force in each buffer solution tested, and acetylcholine-induced contractility was always better preserved than histamine-induced contractility. The best preservation (approximately 65%) was obtained in a potassium-rich buffer solution. The absence of calcium and magnesium from the incubating medium had no influence, whereas the presence of EDTA significantly affected functional recovery. It is difficult to compare our results with those reported by others because of multiple methodological differences. However, it seems that previous results can be improved by changing the freezing rate and the composition of the incubating and cryoprotecting medium.

Histamine and histamine receptors: behavioral thermoregulation in the salamander Necturus maculosus

Low doses (0.01, 0.1 mg/kg, i.p.) of histamine (HA) caused selection of significantly lower temperatures, and higher doses (0.5, 1.0 mg/kg) increased temperatures by mudpuppies in linear thermal gradients. Injection of the HA precursor, L-histidine (500 mg/kg) produced an increase in the temperatures selected. Results from injections of HA H1-receptor agonist (2-pyridylethylamine) and antagonist (pyrilamine), and H2-receptor agonist (dimaprit) and antagonist (cimetidine) had significant effects on thermoregulation; H1-receptors may mediate behavioral hyperthermia and H2-receptors behavioral hypothermia. Responses to these histaminic compounds are significantly influenced by the time of day at which the responses are measured and by season and acclimation temperature. The equivalent behavioral responses in both endotherms and ectotherms to agents which produce physiological hyperthermia and hypothermia are probably behavioral hypothermia ("cold seeking") and behavioral hyperthermia ("heat seeking"), respectively.

Histamine-mediated acetylcholine release in the guinea-pig ileum

The isometric contraction induced by histamine in guinea-pig ileum longitudinal muscle was biphasic in Krebs ([Ca]: 3.36 mM) but monophasic in Tyrode solution ([Ca]: 1.80 mM). The late phase (histamine: 20-400 nM) was common to the two solutions but the early phase (histamine: 2-20 nM) was observed only in Krebs solution. This early phase could be blocked with atropine (0.01-0.2 microM), morphine (0.1, 1 microM), adenosine (5, 20 microM) and tetrodotoxin (0.3 microM) without affecting the late phase. Washout of morphine or adenosine was fast. Neostigmine (100 nM) greatly potentiated the effect of histamine (4 nM) in the early phase, the muscle undergoing almost maximum contraction but also reversible desensitization to doses of histamine less than or equal to 20 nM for as long as 40 min after washout. Beyond this concentration, the preparation responded to increasing doses of histamine as observed in the late phase. It is concluded that low concentrations of histamine that have no observable direct effect on muscle contractility release acetylcholine in the presence of [Ca] 3.36 mM, the early phase being entirely due to release of endogenous acetylcholine.

Evidence against the temperature-dependent interconversion of histamine H1- and H2-receptors in the guinea-pig ileum

1 The possible temperature-dependent interconversion of histamine H1- and H2-receptors in the guinea-pig ileum suggested from previous studies was re-investigated by use of new and selective H2-receptor agonists and antagonists. 2 Chlorpheniramine, and H1-blocker, caused a rightward shift of the cumulative histamine dose-response curve at both 37 degrees C and 12 degrees C. Conversely cimetidine and tiotidine, two H2-receptor blockers, were ineffective at both temperatures. Metiamide behaved as a non competitive antagonist at 12 degrees C but only in very high concentrations. 3 Dimaprit and impromidine, two selective H2-receptor agonists, were inactive at both 37 degrees C and 12 degrees C when given alone, whereas at both temperatures they elicited the already described relaxation of the contractions induced by histamine. 4 Similar results were obtained on the guinea-pig whole ileum and on the longitudinal muscle strip: this indicates a lack of interference of the circular smooth muscle. 5 Our results allow us to conclude that no temperature-dependent interconversion of histamine H1- and H2-receptors occurs in the guinea-pig ileum.

Effects of dithiothreitol on the isolated rabbit mesenteric artery

Rabbit mesenteric artery strips exposed to 10(-3) M dithiothreitol (DTT) were contracted with a series of concentrations of histamine and 2-pyridylethylamine (PEA). DTT exposure increased the sensitivity to histamine 100-fold but increased the sensitivity to PEA only 4-fold. DTT did not reduce dimaprit-induced relaxations, but reduced histamine-induced relaxations. Following a high concentration of histamine (10(-3) M), DTT itself produced a sustained, slowly developing contraction (29 +/- 6.8% of the maximal contraction) relaxed by 7 X 10(-6) M mepyramine but not by 10(-6) M phentolamine. Metiamide (3 x 10(-5) M) potentiated DTT-induced contractions (29 +/- 6.8 before, 57 +/- 7.5% after metiamide, as a percent of maximal contraction). Changing the bathing fluid and repeating DTT exposure slowly relaxed previously contracted strips. DTT did not prevent the increase in sensitivity of relaxant histamine receptors on exposure to cold. We conclude that DTT, in addition to potentiating histamine H1-receptor responses, releases histamine presumably from non-mast cell pools when they are loaded with a high concentration of exogenous histamine.

Histamine H2 receptors in rat renal glomeruli

The aim of this study was to demonstrate histamine-H2 receptors in glomeruli isolated from rat renal cortex and to correlate binding to stimulation by histamine of glomerular cyclic AMP concentration. Binding studies were performed at 10-12 degrees C using [3H]cimetidine as a tracer. Specificity of binding relies on the following: inhibition of [3H]cimetidine binding by the unlabelled drug, other H2-antagonists and agonists in contrast with the very weak inhibitory effects of H1 agonists and antagonists; reversibility of steady-state binding after addition of unlabelled drug; half inhibition of the glomerular cyclic AMP response to histamine at concentrations of cimetidine close to the KD value derived from the binding studies (3 microM); calculated KD value in agreement with the therapeutical concentration of cimetidine and the physiological concentration of histamine. [3H]Cimetidine binding concentration of cimetidine and the physiological concentration of histamine. [3H]Cimetidine binding strikingly increased in the presence of copper chloride (20-300 microM) due to an increase both in number of sites and affinity. However this greater binding did not influence either the inhibitory effect of cimetidine on histamine-induced glomerular cyclic AMP concentration or the stimulatory effect of histamine itself. [3H]Cimetidine binding was temperature-dependent since it progressively diminished from 0 to 37 degrees. This was not due to [3H]cimetidine degradation as shown by thin layer chromatography but rather to a change in drug-receptor interaction at higher temperatures. Glumerular concentration of cyclic AMP increased progressively in the presence of histamine (0.1-1000 microM). This stimulatory effect was markedly inhibited by H2 antagonists. These data demonstrate the presence in rat glomeruli of H2 receptors linked to adenylate cyclase.

Histamine receptors in the guinea pig ileum

Histamine and some related compounds acting selectively on H2-or H1 receptors were tested for their ability to contract the guinea pig ileum, in the usual whole ileum preparation and in the longitudinal muscle preparation. The concentrations elicited by histamine in both kinds of preparations were not potentiated by cimetidine or metiamide and were not inhibited by administration of H2 receptor selective agonists in doses which were subthreshold for contracting the guinea pig ileum; higher doses of the H2 agonists could actually potentiate the effect of histamine. The results obtained suggest that H2 receptors with relaxing effect do not occur in the guinea pig ileum or at least that they are not involved in the contraction of the longitudinal muscle layers. The possibility that a sub-type of H2 receptors with properties different from those of the "classical" H2 receptors so far known, exists in the guinea pig ileum, cannot be excluded.

Fundamental difference between the molecular interactions of agonists and antagonists with the beta-adrenergic receptor

Antagonist binding to the beta-adrenergic receptor is largely entropy driven, with only a small enthalpy component. The binding of agonists, on the other hand, is associated with a large decrease in enthalpy which permits a highly unfavourable decrease in entropy. The thermodynamic differences between the binding of agonists and antagonists may provide new insights into the molecular basis for hormone stimulation of adenylate cyclase activity.

Thyroid hormone-dependent interconversion of myocardial alpha- and beta-adrenoceptors in the rat

1 The effects of thyroid state on the properties of adrenoceptors mediating inotropic and chronotropic responses of the rat heart were assessed on the basis of the relative potencies of alpha- and beta-adrenoceptor agonists, the effects of alpha- and beta-adrenoceptor antagonists and the tissue uptake of [3H]-phenoxybenzamine ([3H]-PB). 2 In isolated, electrically driven left atria the ratio of the inotropic potencies of isoprenaline and phenylephrine and the inhibitory potency of propranolol (40nM-4 muM) were significantly reduced after thyroidectomy and were moderately increased after thyroxine treatment of control rats. 3 Block of inotropic responses to noradrenaline and to phenylephrine by PB (7.3 nM-7.3 muM) and the tissue uptake of [3H]-PB were significantly greater in preparations from thyroidectomized than in those from control or from thyroxine treated rats. alpha-Adrenoceptor inhibition by phentolamine (0.26-2.6 muM) also increased after thyroidectomy, and phentolamine effectively protected alpha-adrenoceptors from block by and binding of [3H]-PB. 4 The beta1-receptor antagonist H 93/26 (0.1 muM) significantly potentiated alpha-adrenoceptor blockade by PB in hypothyroid but not in control preparations. 5 In spontaneously beating right atria the chronotropic potency of agonists and the effects of antagonists were altered in the same way as were inotropic responses and the slope of the agonist concentration-response curves were significantly reduced after thyroidectomy. Effects of agonists and antagonists were not significantly influenced by thyroxine treatment. 6 Changes in the effects and tissue uptake of sympathomimetic drugs observed after thyroidectomy were reversed to or beyond control levels by thyroid hormone treatment of thyroidectomized animals. 7 The results presented are interpreted as indicating a thyroid hormone-dependent interconversion of myocardial alpha- and beta-adrenoceptors. It is suggested that this interconversion is similar to that observed earlier in frog hearts at different temperatures, and that both effects may reflect an allosteric transition between two forms of a single basic structure.

Action of H1 and H2 inhibitors on the response of histamine sensitive adenyly cyclase from guinea-pig mucosa

In the guinea-pig, it has been shown that homogenates of mucosa from the fundus contain an adenylyl cyclase system that is activated by histamine as well as by prostaglandins PGE1 and PGA1. The effects of burimamide, an H2-inhibitor, and mepyramine and chlorpheniramide, both H1-inhibitors, were tested. Both H1 and H2 inhibitors behaved kinetically as competitive inhibitors of histamine, but the Km derived for burimamide (2.5 - 4.1 . 10(-5)) was significantly lower than that for either chlorpheniramine (0.9 - 1.9 . 10(-4)) or mepyramine (1.3 - 1.4 . 10(-4)). On the other hand none of the three inhibitors influenced the cyclase activation by PGE1 and PGA1. These results suggest that there are at least two types of receptors in the preparation studied, one responsive to histamine and the other to the prostaglandins, and that the specificity of H1- and H2-receptors is not absolute in the broken cell preparation.

The effects of metiamide on gastric secretion and stress ulceration in rats

The effects of metiamide, a histamine H2 blocker, on gastric secretion and ulcer formation in stressed pylorus-occluded rats were investigated. Metiamide, like atropine, significantly reduced the volume of gastric secretion and total acid output in unrestrained pylorus-occluded rats. Both drugs produced greater decreases in the volumes of gastric secretion in stressed rats than in their corresponding unrestrained groups. Stress itself reduced both parameters. Metiamide, like atropine, significantly reduced the incidence of gastric stress ulcers. When given together these two drugs did not provide greater protection. The results obtained with metiamide indicate that histamine plays a role in basal gastric secretion and in the pathogenesis of stress ulcers. As no correlation between gastric acid secretion and ulcer formation was demonstrated in this study, it is suggested that H2 receptors may also be involved in gastric motility. However, the possibility that metiamide could exert its ulcer-protecting effects through other mechanisms cannot yet be excluded.