Article date: July 1996
By: David Sugden, Naveed Anwar, David C. Klein, in Volume 118, Issue 5, pages 1246-1252
The pharmacological characteristics of α1‐adrenoceptor binding sites in rat pineal gland membranes, detected by use of a selective α1‐adrenoceptor antagonist ([125I]‐iodo‐2‐[β‐(4‐hydroxyphenyl) ethylaminomethyl]tetralone, [125I]‐HEAT), were investigated with the alkylating agent, chloroethylclonidine (CEC), and in competition experiments with a number of adrenoceptor agonists and antagonists.
Chloroethylclonidine (CEC) treatment (10 μm, 10 min) of rat pineal membranes inactivated ∼ 70% of specific [125I]‐HEAT binding sites. Higher concentrations of CEC (up to 100 μm) or longer treatment periods (upto 40 min) were no more effective.
Adrenoceptor agonists and antagonists competitively inhibited [125I]‐HEAT binding with Hill coefficients close to unity indicating a single α1‐adrenoceptor subtype is present. The affinity (Ki of subtype selective agonists (oxymetazoline, SDZ NVI‐085) and antagonists (5‐methylurapidil, WB4101, benoxathian, phentolamine) was consistent with binding to an α1B‐adrenoceptor subtype.
The (−)− and (+)‐enantiomers of niguldipine had an equal and low affinity for α1‐adrenoceptor binding sites both in untreated (log Ki − 6.66 and −6.90 respectively) and CEC‐treated membranes in which ∼ 70% of sites had been inactivated (log Ki − 6.41 and −6.86 respectively). This indicates that the small proportion of α1‐adrenoceptors insensitive to CEC are not α1A‐adrenoceptors.
mRNA was isolated from rat pinealocytes, cDNA was synthesized and then amplified by the polymerase chain reaction with α1‐adrenoceptor subtype specific primers. These experiments identified both α1A‐ and α1B‐adrenoceptor mRNA, but not α1D‐mRNA in rat pinealocytes, although all three adrenoceptor subtypes were readily identified in rat brain cortex.
These data indicate that although both α1A‐ and α1B‐adrenoceptor mRNAs are present in the pineal the major subtype of α1‐adrenoceptor expressed is the α1B.
The pharmacological characteristics of α1‐adrenoceptor binding sites in rat pineal gland membranes, detected by use of a selective α1‐adrenoceptor antagonist ([125I]‐iodo‐2‐[β‐(4‐hydroxyphenyl) ethylaminomethyl]tetralone, [125I]‐HEAT), were investigated with the alkylating agent, chloroethylclonidine (CEC), and in competition experiments with a number of adrenoceptor agonists and antagonists.
Chloroethylclonidine (CEC) treatment (10 μm, 10 min) of rat pineal membranes inactivated ∼ 70% of specific [125I]‐HEAT binding sites. Higher concentrations of CEC (up to 100 μm) or longer treatment periods (upto 40 min) were no more effective.
Adrenoceptor agonists and antagonists competitively inhibited [125I]‐HEAT binding with Hill coefficients close to unity indicating a single α1‐adrenoceptor subtype is present. The affinity (Ki of subtype selective agonists (oxymetazoline, SDZ NVI‐085) and antagonists (5‐methylurapidil, WB4101, benoxathian, phentolamine) was consistent with binding to an α1B‐adrenoceptor subtype.
The (−)− and (+)‐enantiomers of niguldipine had an equal and low affinity for α1‐adrenoceptor binding sites both in untreated (log Ki − 6.66 and −6.90 respectively) and CEC‐treated membranes in which ∼ 70% of sites had been inactivated (log Ki − 6.41 and −6.86 respectively). This indicates that the small proportion of α1‐adrenoceptors insensitive to CEC are not α1A‐adrenoceptors.
mRNA was isolated from rat pinealocytes, cDNA was synthesized and then amplified by the polymerase chain reaction with α1‐adrenoceptor subtype specific primers. These experiments identified both α1A‐ and α1B‐adrenoceptor mRNA, but not α1D‐mRNA in rat pinealocytes, although all three adrenoceptor subtypes were readily identified in rat brain cortex.
These data indicate that although both α1A‐ and α1B‐adrenoceptor mRNAs are present in the pineal the major subtype of α1‐adrenoceptor expressed is the α1B.
DOI: 10.1111/j.1476-5381.1996.tb15530.x
View this article