Histamine receptor blocking effects of cimetidine in the airways

Effects of histamine H1 receptors on the feeding and drinking patterns in pygmy goats

The goal of these experiments was to determine which histamine receptors are involved in the relationship between drinking and feeding in ruminants. To this end, the effects of the histamine receptor antagonists dexbrompheniramine (H1 receptor antagonist), cimetidine (H2 receptor antagonist), and terfenadine (H1 receptor antagonist) on feeding and drinking patterns of pygmy goats were investigated. Two experiments using dexbrompheniramine [1 and 2 mg/kg of body weight (BW)0.75], two experiments using cimetidine (16 and 32 mg/kg of BW0.75), and two experiments using terfenadine (5 and 11.5 mg/kg of BW0.75) were performed to assess the type and location (periphery or central nervous system) of the histamine receptors involved in the mediation of prandial drinking by pygmy goats. The H1 receptor antagonists dexbrompheniramine (2 mg/kg of BW0.75) and terfenadine (11.5 mg/kg of BW0.75) significantly reduced water intake, but cumulative feed intake did not change. Consequently, the ratio of water intake to feed intake decreased. In contrast, the H2 receptor antagonist did not affect either water or feed intake. Dexbrompheniramine at 2 mg/kg of BW0.75 and terfenadine at 11.5 mg/kg of BW0.75 also decreased draft frequency and decreased the water intake associated with meals. Results showed that blockage of peripheral H1 histamine receptors attenuates the association between water and feed intake in pygmy goats. Therefore, the stimulating effect of feed intake on water intake appears to depend on activation of peripheral H1 histamine receptors.

The response of cat airways to histamine in vivo and in vitro

The effects of histamine have been examined in anaesthetized cats and on cat cat isolated lung parenchyma strip. Histamine infused intravenously for 2 min produced a small and inconsistent effect on central airways and a small but consistent constriction of peripheral airways. Histamine bronchoconstriction of the central airways was unmasked by non-selective and beta 2-adrenoceptor blockade but not by beta 1-adrenoceptor blockade. This bronchoconstriction was antagonized by atropine but not by cimetidine or prazosin. Bronchoconstriction of the peripheral airways was not affected in a dose-related manner by beta-adrenoceptor blockade. The bronchoconstriction was antagonized by mepyramine but not by atropine or prazosin. beta-Adrenoceptor antagonists produced a bell-shaped dose-response curve on histamine contractions in cat isolated lung parenchyma strip. Strips of lung parenchyma obtained from reserpine-treated cats produced a larger contraction to histamine which was not potentiated by propranolol. It is concluded that in the central airways, histamine bronchoconstriction produced by an action on irritant receptors is masked by an action on beta 2-adrenoceptors of catecholamines released locally and from the adrenal glands. In the peripheral airways, histamine bronchoconstriction is mediated by H1-receptors and beta 2-adrenoceptor blockade may either potentiate or antagonize the histamine response depending on the concentration.

Functional depression of H2 histamine receptors in sheep with experimental allergic asthma

We tested the hypothesis that airway hyperresponsiveness to histamine in the allergic sheep is related to functional depression of H2 histamine receptors. Thirteen of 32 sheep responded with bronchoconstriction to inhaled Ascaris suum antigen (allergic sheep), and the remainder served as controls (nonallergic sheep). In the allergic sheep, 50 and 100 breaths of 5% histamine solution increased mean pulmonary resistance (RL) to 235% and 438% of baselines, respectively. The corresponding values in nonallergic sheep were 200% and 211%, indicating a greater response to the higher dose of histamine in allergic sheep. Selective H1-receptor stimulation with 50 breaths of histamine (pretreatment with the H2-receptor antagonist metiamide) failed to enhance the effect of histamine in allergic sheep (mean RL increased to 239% of baseline) whereas it enhanced the histamine response in nonallergic sheep (RL increased to 438% of baseline). Selective H2-receptor stimulation (pretreatment with the H1-receptor antagonist chlorpheniramine) caused histamine to decrease RL by 31% in the nonallergic sheep group; it blocked but did not reverse the histamine effect in the allergic sheep. Similar observations were made in a different group of animals when selective H1- or selective H2-receptor stimulation was produced by 100 breaths of histamine. The cutaneous wheal response to intradermal histamine dilutions of 0.0001. 0.001, 0.01, and 1 mg/ml was similar in both groups. In nonallergic sheep, both chlorpheniramine and metiamide blunted the cutaneous wheal response. In allergic animals, only chlorpheniramine blunted the cutaneous wheal response, whereas metiamide was without effect. We conclude that airway hyperresponsiveness to histamine in allergic sheep is related to a functional depression of H2 receptors and that such a defect is observed both in the airways as well as in the skin.