Comparison of guinea-pig, bovine and rat α1-adrenoceptor subtypes

Abstract

1. To elucidate a possible role of species differences in the classification of α1-adrenoceptor subtypes, we have characterized the α1-adrenoceptors in guinea-pig spleen, kidney and cerebral cortex and in bovine cerebral cortex using concentration-dependent alkylation by chloroethylclonidine and competitive binding with 5-methylurapidil, methoxamine, (+)-niguldipine, noradrenaline, oxymetazoline, phentolamine, SDZ NVI-085, tamsulosin and (+)-tamsulosin. Rat liver α(1B)-adrenoceptors were studied for comparison. Chloroethylclonidine-sensitivity and (+)-niguldipine affinity were also compared at cloned rat and bovine α(1a)-adrenoceptors. 2. Chloroethylclonidine concentration-dependently inactivated α1-adrenoceptors in all five tissues. While chloroethylclonidine inactivated almost all α1-adrenoceptors in rat liver and guinea-pig kidney and brain, 20-30% of α1-adrenoceptors in guinea-pig spleen and bovine brain were resistant to alkylation by 10 μM chloroethylclonidine. With regard to concentration-dependency guinea-pig kidney and brain were approximately 10 fold less sensitive than guinea-pig spleen or rat liver. 3. In rat liver, all drugs tested competed for [3H]-prazosin binding with steep and monophasic curves. Drug affinities were relatively low and resembled most closely those of cloned rat α(1b)-adrenoceptors. 4. In guinea-pig spleen, all drugs tested competed for [3H]-prazosin binding with steep and monophasic curves. Drug affinities were relatively low and resembled most closely those of cloned rat α(1b)-adrenoceptors. 5. In guinea-pig kidney most drugs tested competed for [3H]-prazosin binding with steep and monophasic curves and had relatively low drug affinities close to those of cloned rat α(1b)- and α(1d)-adrenoceptors. However, noradrenaline and tamsulosin had consistently biphasic competition curves recognizing 36-39% high and 61-64% low affinity sites. 6. In guinea-pig cerebral cortex, all drugs tested competed for [3H]-prazosin binding with shallow and biphasic curves. While most drugs recognized approximately 25% high affinity sites, tamsulosin and noradrenaline recognized approximately 50% high affinity sites. Drug affinities at the high and low affinity sites except those for tamsulosin and noradrenaline resembled those at cloned α(1a)- and α(1b)-adrenoceptors, respectively. 7. In bovine cerebral cortex all drugs tested except for noradrenaline competed for [3H]-prazosin binding with shallow and biphasic curves. All drugs recognized approximately 70% high affinity sites. Drug affinities at the high and low affinity sites resembled those at cloned α(1a)- and α(1b)-adrenoceptors, respectively. Noradrenaline competition curves in bovine cerebral cortex were steep and monophasic. 8. When cloned rat and bovine α(1a)-adrenoceptors transiently expressed in COS cells were studied in a direct side-by-side comparison, both species homologues had similar chloroethylclonidine-sensitivity and (+)-niguldipine affinity. 9. We conclude that properties of bovine α(1A)- and α(1B)-adrenoceptors are very similar to those of other species such as rat. α1-Adrenoceptor subtypes in guinea-pigs resemble α(1A)- and α(1B)-adrenoceptors in other species but chloroethylclonidine sensitivity and competition binding profiles of noradrenaline and tamsulosin are not compatible with previously established α1-adrenoceptor subtype classification.