COMPARISON OF GUINEA-PIG, BOVINE AND RAT ALPHA(1)-ADRENOCEPTOR SUBTYPES

1 To elucidate a possible role of species differences in the classific ation of alpha(1)-adrenoceptor subtypes, we have characterized the alp ha(1)-adrenoceptors in guinea-pig spleen, kidney and cerebral cortex a nd in bovine cerebral cortex using concentration-dependent alkylation by chloroethylclonidine and competitive binding with 5-methylurapidil, methoxamine, (+)-niguldipine, noradrenaline, oxymetazoline, phentolam ine, SDZ NVI-085, tamsulosin and (+)-tamsulosin. Rat liver alpha(1B)-a drenoceptors were studied for comparison. Chloroethylclonidine-sensiti vity and (+)-niguldipine affinity were also compared at cloned rat and bovine alpha(1a)-adrenoceptors. 2 Chloroethylclonidine concentration- dependently inactivated alpha(1)-adrenoceptors in all five tissues. Wh ile chloroethylclonidine inactivated almost all alpha(1)-adrenoceptors in rat liver and guinea-pig kidney and brain, 20-30% of alpha(1)-adre noceptors in guinea-pig spleen and bovine brain were resistant to alky lation by 10 mu M chloroethylclonidine. With regard to concentration-d ependency guinea-pig kidney and brain were approximately 10 fold less sensitive than guinea-pig spleen or rat liver. 3 In rat liver, all dru gs tested competed for [H-3]-prazosin binding with steep and monophasi c curves. Drug affinities were relatively low and resembled most close ly those of cloned rat alpha(1b)-adrenoceptors. 4 In guinea-pig spleen , all drugs tested competed for [H-3]-prazosin binding with steep and monophasic curves. Drug affinities were relatively low and resembled m ost closely those of cloned rat alpha(1b)-adrenoceptors. 5 In guinea-p ig kidney most drugs tested competed for [H-3]-prazosin binding with s teep and monophasic curves and had relatively low drug affinities clos e to those of cloned rat alpha(1b)- and alpha(1d)- adrenoceptors. Howe ver, noradrenaline and tamsulosin had consistently biphasic competitio n curves recognizing 36-39% high and 61-64% low affinity sites. 6 In g uinea-pig cerebral cortex, all drugs tested competed for [H-3]-prazosi n binding with shallow and biphasic curves. While most drugs recognize d approximately 25% high affinity sites, tamsulosin and noradrenaline recognized approximately 50% high affinity sites. Drug affinities at t he high and low affinity sites except those for tamsulosin and noradre naline resembled those at cloned alpha(1a)- and alpha(1b)-adrenoceptor s, respectively. 7 In bovine cerebral cortex all drugs tested except f or noradrenaline competed for [H-3]-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 th ose at cloned alpha(1a)- and alpha(1b)-adrenoceptors, respectively. No radrenaline competition curves in bovine cerebral cortex were steep an d monophasic. 8 When cloned rat and bovine alpha(1a)-adrenoceptors tra nsiently expressed in COS cells were studied in a direct side-by-side comparison, both species homologues had similar chloroethylclonidine-s ensitivity and (+)-niguldipine affinity. 9 We conclude that properties of bovine alpha(1A)- and alpha(1B)-adrenoceptors are very similar to those of other species such as rat. alpha(1)-Adrenoceptor subtypes in guinea-pigs resemble alpha(1A)- and alpha(1B)-adrenoceptors in other s pecies but chloroethylclonidine sensitivity and competition binding pr ofiles of noradrenaline and tamsulosin are not compatible with previou sly established alpha(1)-adrenoceptor subtype classification.