Genetic heterogeneity of histamine H2‐receptors in the mouse vas deferens

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Abstract
The ability of histamine to inhibit the overall contractile (‘twitch’) response of the isolated vas deferens of the mouse to electrical field stimulation (64 V pulse, 1 ms pulse width, frequency 0.2 Hz) was studied in nine inbred mouse strains. The strains were also characterized in terms of the potency of the histamine H2‐receptor antagonist cimetidine in its inhibition of histamine‐mediated effects. An apparently bimodal inter‐strain variation (8−10 fold) in both characteristics was encountered, with three strains (SWR, A2G and C57BL/10ScSn) relatively sensitive (S) to both agonist and antagonist actions, and six (C3H, A, C57/BL6, DBA/2, Balb/C and 129/Sv) relatively insensitive (IS). These strain differences were independent of extracellular calcium concentration in the range 1.25−5 mm, and also independent of the frequency of tissue stimulation over the range 0.2−6.4 Hz. Representative S (SWR and A2G) and IS (DBA/2 and C3H) mouse vasa were also characterized in terms of their sensitivity to the agonist actions of dimaprit and the antagonist actions of tiotidine. In the S strain tissues, dimaprit produced 50% inhibition of the twitch response at 4.6−1.8 μm (mean ± s.d.) and was able to elicit complete inhibition of the twitch response at concentrations greater than 100 μm, whereas 48.7 ± 11.9 μm dimaprit was required to produce 50% inhibition of the twitch response in tissues from IS mice. In addition, the agonist actions of dimaprit were incomplete in the latter tissues, the drug eliciting no more than 75% inhibition of the twitch response at concentrations in the range 300−1000 μm. Tiotidine produced competitive antagonism of the actions of both histamine and dimaprit, the strain differences being of the same magnitude as those observed for cimetidine. Mating of a representative S (SWR) and IS (129/Sv) strain produced F1 mice with intermediate histamine and cimetidine sensitivities relative to the parental strains. A backcross of male F1 to female IS mice produced progeny displaying a range of histamine and cimetidine sensitivities representative of those seen in tissues from F1 and IS parental animals, however, the data were not bimodal. Thus, the backcross data provided no evidence to support single gene inheritance of histamine sensitivity and might suggest that more than one gene is responsible for these differences between S and IS mice.