H1 and H2 receptors mediate postexercise hyperemia in sedentary and endurance exercise-trained men and women.

In sedentary individuals, H(1) receptors mediate the early portion of postexercise skeletal muscle hyperemia, whereas H(2) receptors mediate the later portion. It is not known whether postexercise hyperemia also presents in endurance-trained individuals. We hypothesized that the postexercise skeletal muscle hyperemia would also exist in endurance-trained individuals and that combined blockade of H(1) and H(2) receptors would abolish the long-lasting postexercise hyperemia in trained and sedentary individuals. We studied 28 sedentary and endurance trained men and women before and through 90 min after a 60-min bout of cycling at 60% peak O(2) uptake on control and combined H(1)- and H(2)-receptor antagonist days (fexofenadine and ranitidine). We measured arterial pressure (brachial auscultation) and femoral blood flow (Doppler ultrasound). On the control day, femoral vascular conductance (calculated as flow/pressure) was elevated in all groups 60 min after exercise (sedentary men: Delta86 +/- 35%, trained men, Delta65 +/- 18%; sedentary women, Delta61 +/- 19%, trained women: Delta59 +/- 23%, where Delta is change; all P < 0.05 vs. preexercise). In contrast, on the histamine antagonist day, femoral vascular conductance was not elevated in any of the groups after exercise (sedentary men: Delta21 +/- 17%, trained men: Delta9 +/- 5%, sedentary women: Delta19 +/- 4%, trained women: Delta11 +/- 11%; all P > 0.16 vs. preexercise; all P < 0.05 vs. control day). These data suggest postexercise skeletal muscle hyperemia exists in endurance trained men and women. Furthermore, histaminergic mechanisms produce the long-lasting hyperemia in sedentary and endurance-trained individuals.     

Characterization of thromboxane A2/prostaglandin H2 receptors and histamine H1 receptors in cultured guinea-pig tracheal smooth-muscle cells.

We characterized thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors and histamine H1 receptors in Guinea-pig cultured tracheal smooth-muscle cells (TSMC). [3H]SQ 29,548 (a TXA2 antagonist)-binding sites were saturable and a high affinity with a dissociation constant of 6.2 +/- 0.60 nM (mean +/- S.E.) and a receptor density of 46 +/- 4.6 fmol/10(6) cells. [3H]SQ 29548 binding was completely inhibited by TXA2 mimetics or antagonists. Intracellular calcium concentration ([Ca2+]i) in TSMC was increased with U46619 stimulation and the increase was attenuated by TXA2 antagonists, the potencies of which correlated with those inhibiting the activities of the [3H]SQ 29548 binding. [3H]Mepyramine (a H1 antagonist)-binding sites were also present in TSMC. [3H]Mepyramine had a single class of low-affinity-binding sites with a dissociation constant of 2.6 +/- 0.081 microM and a receptor density of 10.6 +/- 0.11 nmol/mg protein. [3H]Mepyramine binding in TSMC membrane was inhibited by H1 antagonists, but not by H2 antagonists. The inhibition constants of mepyramine in TSMC were 910-times lower than those in tracheal membranes. In contrast, the histamine-induced increase in [Ca2+]i in TSMC was inhibited in the presence of low concentrations of H1 antagonists. All these observations provide evidence that TXA2/PGH2 receptors, mepyramine-binding sites and/or H1 receptors are expressed in cultured TSMC.     

A role for histamine and histamine H2-receptors in non-opiate footshock-induced analgesia

Scrambled DC current applied to the hind paws of rats caused an analgesic response that was inhibited by the histamine H2-receptor antagonists cimetidine, ranitidine and oxmetidine, but not by high doses of naloxone (the opiate antagonist), or other transmitter receptor antagonists. In contrast, AC current applied to all paws produced analgesia that was blocked by naloxone, but not cimetidine, showing the independence of these systems. These findings indicate a specific role for histamine and H2-receptors as mediators of endogenous non-opiate analgesia. In addition, a combination of cimetidine and naloxone did not abolish either form of footshock analgesia, implying the existence of a non-opiate, non-H2, endogenous pain-relieving system. These results also suggest that drugs capable of penetrating the brain and stimulating H2-receptors might have analgesic properties.     

H2 histaminic receptors in rat cerebral cortex. 1. Binding of [3H]histamine

Saturable binding of [3H]histamine in equilibrium with homogenates of rat cerebral cortex reveals Hill coefficients between 0.4 and 1.0, depending upon the conditions. Data from individual experiments are well described assuming one or two classes of sites. Only the sites of higher affinity (KP1 = 3.9 +/- 0.5 nM) are observed when binding is measured by isotopic dilution at a low concentration of the radioligand (less than 1.5 nM) in the presence of magnesium or by varying the concentration of the radioligand. The sites of lower affinity (KP2 = 221 +/- 26 nM) appear during isotopic dilution at higher concentrations of the radioligand or at lower concentrations either upon the addition of guanylyl imidodiphosphate (GMP-PNP) or upon the removal of magnesium. Estimates of the second- and first-order rate constants for association and dissociation of [3H]histamine agree well with KP1. Apparent capacities corresponding to KP1 and KP2 are of the order of 100 ([R1]t) and 1300 pmol/g of protein ([R2]t), respectively. Simple interconversion cannot account for the changes in binding that occur upon adding GMP-PNP or removing magnesium, since the increase in [R2]t exceeds the decrease in [R1]t. Moreover, the apparent amount of high-affinity complex exhibits a biphasic dependence on the concentration of [3H]histamine; an increase at low concentrations is offset by a decrease that occurs at higher concentrations. The latter appears to be positively cooperative and concomitant with formation of the low-affinity complex. These and other observations indicate that the binding of histamine is inconsistent with models commonly invoked to rationalize the binding of agonists to neurohumoral receptors. GMP-PNP and magnesium reciprocally alter capacity at the sites of higher affinity, however, and the reduction caused by GMP-PNP reflects a substantial increase in the rate constant for dissociation at the sites that appear to be lost. The sites labeled by [3H]histamine thus reveal the properties of neurohumoral receptors linked to a nucleotide-specific G/F protein.     

3H]mepyramine binding to histamine H1 receptors in bovine retina

The promethazine-sensitive [3H]mepyramine binding was used to determine the presence of histamine H1 receptors in membranes from bovine retina. Specific mepyramine binding to retinal membranes was reversible, saturable and of high affinity. The apparent dissociation constant (KD = 2.2 +/- 0.4 nM) and the density of binding sites (Bmax = 60.9 +/- 5.1 fmol/mg protein), obtained in equilibrium studies, were similar to those found in bovine brain cortex. Binding was stereospecific and the inhibitory potencies of H1 and H2 antagonists indicated that [3H] mepyramine binding sites in the retina have characteristics of H1 receptors.     

Cyclooxygenase and nitric oxide synthase dependence of cutaneous reactive hyperemia in humans

We tested the hypothesis that cyclooxygenases (COXs) or COX products inhibit nitric oxide (NO) synthesis and thereby mask potential effects of NO on reactive hyperemia in the cutaneous circulation. We performed laser-Doppler flowmetry (LDF) with intradermal microdialysis in 12 healthy volunteers aged 19-25 yr. LDF was expressed as the percent cutaneous vascular conduction (%CVC) or as the maximum %CVC (%CVC(max)) where CVC is LDF/mean arterial pressure. We tested the effects of the nonisoform-specific NO synthase inhibitor nitro-L-arginine (NLA, 10 mM), the nonspecific COX inhibitor ketorolac (Keto, 10 mM), combined NLA + Keto, and NLA + sodium nitroprusside (SNP, 28 mM) on baseline and reactive hyperemia flow parameters. We also examined the effects of isoproterenol, a beta-adrenergic agonist that causes prostaglandin-independent vasodilation to correct for the increase in baseline flow caused by Keto. When delivered directly into the intradermal space, Keto greatly augments all aspects of the laser-Doppler flow response to reactive hyperemia: peak reactive hyperemic flow increased from 41 +/- 5 to 77 +/- 7%CVC(max), time to peak flow increased from 17 +/- 3 to 56 +/- 24 s, the area under the reactive hyperemic curve increased from 1,417 +/- 326 to 3,376 +/- 876%CVC(max).s, and the time constant for the decay of peak flow increased from 100 +/- 23 to 821 +/- 311 s. NLA greatly attenuates the Keto response despite exerting no effects on baseline LDF or on reactive hyperemia when given alone. Low-dose NLA + SNP duplicates the Keto response. Isoproterenol increased baseline and peak reactive flow. These results suggest that COX inhibition unmasks NO dependence of reactive hyperemia in human cutaneous circulation.     

The involvement of norepinephrine, neuropeptide Y, and nitric oxide in the cutaneous vasodilator response to local heating in humans.

Presynaptic blockade of cutaneous vasoconstrictor nerves (VCN) abolishes the axon reflex (AR) during slow local heating (SLH) and reduces the vasodilator response. In a two-part study, forearm sites were instrumented with microdialysis fibers, local heaters, and laser-Doppler flow probes. Sites were locally heated from 33 to 40 degrees C over 70 min. In part 1, we tested whether this effect of VCN acted via nitric oxide synthase (NOS). In five subjects, treatments were as follows: 1) untreated; 2) bretylium, preventing neurotransmitter release; 3) N(G)-nitro-L-arginine methyl ester (L-NAME) to inhibit NOS; and 4) combined bretylium + L-NAME. At treated sites, the AR was absent, and there was an attenuation of the ultimate vasodilation (P < 0.05), which was not different among those sites (P > 0.05). In part 2, we tested whether norepinephrine and/or neuropeptide Y is involved in the cutaneous vasodilator response to SLH. In seven subjects, treatments were as follows: 1) untreated; 2) propranolol and yohimbine to antagonize alpha- and beta-receptors; 3) BIBP-3226 to antagonize Y(1) receptors; and 4) combined propranolol + yohimbine + BIBP-3226. Treatment with propranolol + yohimbine or BIBP-3226 significantly increased the temperature at which AR occurred (n = 4) or abolished it (n = 3). The combination treatment consistently eliminated it. Importantly, ultimate vasodilation with SLH at the treated sites was significantly (P < 0.05) less than at the control. These data suggest that norepinephrine and neuropeptide Y are important in the initiation of the AR and for achieving a complete vasodilator response. Since VCN and NOS blockade in combination do not have an inhibition greater than either alone, these data suggest that VCN promote heat-induced vasodilation via a nitric oxide-dependent mechanism.     

Differential activation of dual signaling responses by human H1 and H2 histamine receptors

Stimulation of human H1 and H2-histamine receptors (HRs) primarily activates signaling pathways to increase intracellular calcium [Ca2+]i and cyclic AMP (cAMP), respectively. Activation of H2-HR in human embryonic kidney (HEK) cells by histamine and dimaprit increases both cAMP formation and [Ca2+]i, as determined by cAMP-scintillation proximity assays and fluorescence imaging plate reader (FLIPR) assays. In HEK cells expressing relatively high levels of H2-HR (Bmax=26 pmol/mg protein), histamine and dimaprit are full agonists in eliciting cAMP responses with pEC50 values of 9.30 and 7.72 that are 1000-fold more potent than their respective pEC50 values of 6.13 and 4.91 for increasing [Ca2+]i. The agonist potencies decrease for both responses at lower H2-HR density (5 pmol/mg protein) and dimaprit exhibits partial agonist behavior for the [Ca2+]i response. The inverse agonists ranitidine and cimetidine more potently inhibit cAMP production in the higher expressing H2-HR line. Histamine also activated both signaling pathways via human H1-HRs highly expressed (Bmax=17 pmol/mg protein) in HEK cells, with a 1000-fold greater potency for [Ca2+]i vs. cAMP responses (pEC50=7.86 and 4.82, respectively). These studies demonstrate a markedly different potency for activation of multiple signaling pathways by H1- and H2-HRs that may contribute to the selectivity of histamine responses in vivo.     

Histamine H3 receptors modulate reactive hyperemia in rat gut.

Reactive hyperemia (RH) is an abrupt blood flow increase following release from mechanical occlusion of an artery, with restoration of intra-arterial pressure. The mechanism of this postocclusion increase in blood flow in the gut is multifactorial. Relaxation of intestinal resistance vessels, observed during RH, may involve myogenic, metabolic, hormonal and neurogenic factors. Evidence exists that histamine is an important endogenous mediator of various functions of the gut, including blood flow. The vascular effects of histamine in the intestinal circulation are due its agonistic action on histamine H1, H2 and H3 receptors. In the present study the hypothesis was tested that peripheral histamine H3 receptors are involved in the mediation of RH in the intestinal circulation. In anesthetized rats, anterior mesenteric artery blood flow (MBF) was determined with ultrasonic Doppler flowmeter, and arterial pressure (AP) was determined with a transducer. The increase in the volume of blood accumulating during RH (RH-volume), the peak increase of arterial blood flow (RH-peak response) and the duration of the hyperemia (RH-duration) were used to quantify RH after occluding the anterior mesenteric artery for 30, 60 and 120 s. Hyperemia parameters were determined before and after administration of the selective histamine H3 receptor antagonist clobenpropit. Pretreatment with clobenpropit was without any effect on control MBF and AP but significantly reduced most of RH responses. These findings support the hypothesis that histamine H3 receptors do not play any role in the control of intestinal vasculature at basal conditions but these receptors participate in the intestinal hyperemic reaction in response to complete temporal intestinal ischemia.     

Effect of histamine and H1 and H2 receptors antagonists on carbachol-induced wet-dog shakes in rats

Intracerebroventricular administration of carbachol chloride induced a characteristic wet-dog shake response in rats. Histamine did not change the number of wet-dog shakes during a 60 min observation but intensified the number of episodes in the first 30 min of the experiment. Antagonists of H1 (thenalidine and antazoline) and H2 (cimetidine and ranitidine) receptors, attenuated carbachol-induced wet-dog shakes. It may be suggested that inhibition of the central histaminergic structures decreased central cholinergic activity.     

Implication of prostaglandins and histamine H1 and H2 receptors in radiation-induced temperature responses of rats

Exposure of rats to 1-15 Gy gamma radiation (60Co) induced hyperthermia, whereas 20-200 Gy induced hypothermia. Exposure either to the head or to the whole body to 10 Gy induced hyperthermia, while body-only exposure produced hypothermia. This observation indicates that radiation-induced fever is a result of a direct effect on the brain. The hyperthermia due to 10 Gy was significantly attenuated by the pre- or post-treatment with a cyclooxygenase inhibitor, indomethacin. Hyperthermia was also altered by the central administration of a mu-receptor antagonist naloxone but only at low doses of radiation. These findings suggest that radiation-induced hyperthermia may be mediated through the synthesis and release of prostaglandins in the brain and to a lesser extent to the release of endogenous opioid peptides. The release of histamine acting on H1 and H2 receptors may be involved in radiation-induced hypothermia, since both the H1 receptor antagonist, mepyramine, and H2 receptor antagonist, cimetidine, antagonized the hypothermia. The results of these studies suggest that the release of neurohumoral substances induced by exposure to ionizing radiation is dose dependent and has different consequences on physiological processes such as the regulation of body temperature. Furthermore, the antagonism of radiation-induced hyperthermia by indomethacin may have potential therapeutic implications in the treatment of fever resulting from accidental irradiations.     

Importance of histamine H2 receptors in restraint-morphine interactions

The effects of the brain-penetrating H₂ antagonist zolantidine (ZOL, 3 mg/Kg, S.c.) were studied on morphine (MOR, 4 mg/Kg, S.c.) antinociception (tail flick test) in the presence and absence of previous restraint stress. Animals were handled for 3 days (to reduce handling stress), restrained for 1 hr or handled on day 4 and tested 24 hrs later. As found previously, restraint enhanced the intensity and duration of MOR antinociception. ZOL potentiated MOR antinociception in handled, non-restrained animals, but inhibited MOR action in restrained animals. In contrast, ZOL had no effects on nociceptive responses in either handled or stressed subjects in the absence of MOR. The data suggest that, in the absence of restraint, brain HA acts at the H₂ receptor to inhibit MOR antinociception. In contrast, when an animal has been previously restrained, HA enhances MOR antinociception. Thus, brain HA appears to mediate the restraintinduced potentiation of MOR antinociception. Taken with previous results, the present findings suggest that in the presence of MOR, brain HA can provide bidirectional modulation of nociception. The direction of the modulation seems to depend upon the stress experience of the animal.     

Involvement of histamine H1 and H2 receptors in hypothermia induced by ionizing radiation in guinea pigs

Radiation-induced hypothermia was examined in guinea pigs. Exposure to the head alone or whole-body irradiation induced hypothermia, whereas exposure of the body alone produced a small insignificant response. Systemic injection of disodium cromoglycate (a mast cell stabilizer) and cimetidine (H2-receptor antagonist) had no effect on radiation-induced hypothermia, whereas systemic and central administration of mepyramine (H1-receptor antagonist) or central administration of disodium cromoglycate or cimetidine attenuated it, indicating the involvement of central histamine through both H1 and H2 receptors in this response. Serotonin is not involved, since the serotonin antagonist methysergide had no effect on radiation-induced hypothermia. These results indicate that central histaminergic systems may be involved in radiation-induced hypothermia.     

Solubilization, separation, and partial characterization of histamine H1 and H2 receptors from calf thymocyte membranes.

Histamine membrane receptors are defined as either H1 (blocked by diphenhydramine-like antagonists) or H2 (blocked by cimetidine-like agents). We now report the solubilization, separation, and partial characterization of specific H1 and H2 membrane receptors from calf thymocytes. Membrane fragments were incubated with [3H]histamine either alone or with unlabeled histamine, diphenhydramine, or cimetidine. Maximal specific binding occurred with incubation at 37 degrees C for 2 h at a concentration of 5 x 10(-6) M [3H]histamine. Labeled receptors were solubilized from membranes with 0.3 M KCl and 1% Nonidet 40. Chromatography of the solubilized labeled receptors on ion exchange columns revealed two classes of receptor. One class bound to DEAE-cellulose and eluted as a sharp peak at 0.15 M NaCl/Pi. The other bound to phosphocellulose and eluted as a sharp peak at 0.55 M NaCl/Pi. Initial incubation of the membranes in the presence of the H1 receptor antagonist diphenhydramine virtually abolished the DEAE-cellulose peak, while incubation with cimetidine, the H2 receptor antagonist, blocked the phosphocellulose peak. We conclude that H1 and H2 histamine receptors are physically separable and can be defined by their ability to bind to either DEAE-cellulose or phosphocellulose.     

H2 histaminic receptors in rat cerebral cortex. 2. Inhibition of [3H]histamine by H2 antagonists

Sites labeled by [3H]histamine in homogenates of rat cerebral cortex reveal a pharmacological specificity typical of H2 receptors. Fourteen H2 antagonists inhibit the specific binding of the radioligand to the same level; Hill coefficients are near or equal to one for five compounds and markedly lower for nine. The binding patterns of individual antagonists (A) are well described by the empirical expression Y = F1K1/(K1 + [A]) + F2K2/(K2 + [A]), in which F1 and F2 sum to 1; F2 is 0 for those drugs that reveal a Hill coefficient of 1. Concentrations of A that reduce specific binding by 50% (IC50) correlate well (r = 0.991; P less than 0.00001) and show good numerical agreement with potencies reported for inhibition of the response to histamine in H2-mediated systems. The correlation is poorer when IC50 is replaced by either K1 (r = 0.973) or K2 (r = 0.921) for those antagonists that reveal both; the antihistaminic activity of the drug thus appears not to be associated preferentially with one or other class of sites. Since F2 varies from 0.16 to 0.60 among those antagonists that discern heterogeneity, the antagonist appears to determine the distribution of sites between the two classes. Moreover, a correlation among antagonists between values of K1 and K2 (r = 0.975; P = 0.00001) suggests that the apparent heterogeneity reflects different conformers within an otherwise homogeneous population. H2 antagonists appear to be noncompetitive with respect to each other and to the radioligand: one antagonist has relatively little effect on the values of K1, K2, and F2 revealed by another; also, estimates of K1 and K2 are independent of the concentration of [3H]histamine between 1.3 and 10 nM, although the radioligand exhibits an apparent dissociation constant of 3.9 nM [Steinberg, G. H., Eppel, J. G., Kandel, M., Kandel, S. I., & Wells, J. W. (1985) Biochemistry (preceding paper in this issue)].     

The binding of [3H]mepyramine to histamine H1 receptors in monkey brain

Several laboratories have reported ligand binding studies using radioactive histamine H1 antagonists to label the H1 receptors in mammalian brain. We have extended these studies to a detailed examination of the binding of [3H]mepyramine to monkey brain and have shown that the distribution is similar to that in man, with specific binding sites being concentrated in the frontal cortex with relatively low binding to the pons and basal ganglia. The binding shows a single saturable component with a KD of about 1 nM and a Hill plot slope close to unity. These observations are the same for all structures tested. Comparison with data from other laboratories suggests that in this species, the histamine receptor is the same as that in peripheral tissues. From Ki values for various ligands and comparison of KD estimates in other species, the receptor seems to be essentially identical to the H1 receptor in central and peripheral tissues of the guinea pig and also to that in human brain. The rat and possibly the dog have minor differences from the monkey in terms of KD values for [3H]mepyramine binding.     

H2 histaminic receptors in rat cerebral cortex. 3. Inhibition of [3H]histamine by H2 agonists

The binding of [3H]histamine to H2 receptors in homogenates of rat cerebral cortex is inhibited by 11 H2 agonists in a characteristic and unique manner. At low concentrations of the radioligand (less than 1.5 nM), the inhibitory profiles of individual agonists (A) are distinctly biphasic; specific binding is well described in most cases by the empirical expression Y = F1K1/(K1 + [A]) + F2K2/(K2 + [A]), in which F1 and F2 sum to 1. Maximal inhibition is the same for all agonists. Since values of F2 vary from 0.42 to 0.90, the agonist appears to determine the equilibrium distribution of receptors between two states of affinity. Ratios of apparent affinity (K2/K1) vary from 204 to 3 090 000, and there is no correlation between values of K1 and K2. Compounds lacking H2 activity, including structural analogues of histamine and dimaprit, reveal a Hill coefficient of 1 and inhibit the radioligand only weakly. For six agonists, values of K2 agree and correlate well (P = 0.00047) with H2 pharmacological potency (EC50) in the guinea pig right atrium; for the others, K2 is less than EC50 by 15-61-fold. Four observations suggest that the inhibition corresponding to F1 is allosteric and cooperative: the dissociation constant of the radioligand appears to vary in the presence of an unlabeled agonist, absolute levels of binding corresponding to F1, as defined by dimaprit, decrease at higher concentrations of [3H]histamine, F1 for dimaprit is reduced from 0.48 to 0.32 by 2-methylhistamine (F1 = 0.27) at a concentration of 20 nM (approximately K1(0.5) K2(0.5) for 2-methylhistamine), but the increase in K1 for dimprit is at least 100-fold less than expected from competitive effects, and 1 equiv of some agonists appears to preclude access of [3H]histamine to more than 1 equiv of receptors, with no evidence that an appreciable fraction of the unlabeled drug is bound. Noncompetitive effects also may account in part for the inhibition corresponding to F2.