Submandibular gland acinar cells express multiple α₁- adrenoceptor subtypes

We evaluated an acinar cell line (SMG-C10) cloned from rat submandibular glands as a possible model for α₁-adrenoceptor regulation of submandibular function. α₁-Adrenoceptors are subdivided into three subtypes called α_1A, α_1B, and α_1D, which can be distinguished from one another by their differential affinity values for subtype-selective α₁- adrenoceptor antagonists. Thus, α₁-adrenoceptor subtypes in SMG-C10 cells were characterized with reverse transcription-polymerase chain reaction (RT-PCR) and [³H]prazosin binding in side-by-side experiments with native submandibular glands. RT-PCR identified mRNAs for α_1A-, α_1B-, and α_1D- adrenoceptors in SMG-C10 cells and submandibular glands. The inhibition of [³H]prazosin binding by 5-methylurapidil (α_1A- selective) was biphasic and fit best to a two-site binding model with 40 ± 8% high (K_iH)- and 60 ± 10% low (K _iL)-affinity binding sites in SMG-C10 cells, and 76% high- and 24% low-affinity binding sites in submandibular glands. Respective K_iH and K_iL values for 5-methylurapidil were 1.9 ± 0.4 and 100 ± 30 nM in SMG-C10 cells and 3.2 ± 0.8 and 170 ± 20 nM in submandibular glands. BMY-7378 [8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- 8-azaspiro[4.5]decane-7,9-dione dihydrochloride (α_1D-selective) ] bound with low affinity in SMG-C10 cells and submandibular glands with K _i values of 81 ± 20 and 110 ± 20 nM, respectively. Chloroethylclondine, an irreversible alkylating agent selective for α_1B adrenoceptors, reduced the density of [³H] prazosin binding sites by 42 and 26% in SMG-C10 and submandibular membranes, respectively. Thus, SMG-C10 cells and submandibular glands are similar in expressing receptor protein for α_1A- and α_1B- adrenoceptor subtypes, establishing SMG-C10 cells as a potential model for α₁-adrenoceptor-mediated secretion.