ALPHA-2A IS THE PREDOMINANT ALPHA-2 ADRENERGIC-RECEPTOR SUBTYPE IN HUMAN SPINAL-CORD

Alpha-2 adrenergic receptors can be subdivided into four subtypes based on their pharmacologic properties. The subtype of alpha-2 adrenergic receptor present in human spinal cord has not been reported previously. The affinities of nine alpha-2 subtype-selective drugs for the alpha-2 adrenergic receptor in human spinal cord homogenates were determined using [H-3]rauwolscine and [H-3]RX821002. These drug affinities (pK(i) values) were highly correlated with those obtained in a tissue or cell line containing only the alpha-2A adrenergic subtype (correlation coefficient of 0.99 and 0.98 for human platelet and HT29 cells, respectively). In contrast, the correlation of pK(i) values for the human spinal cord with tissues or cell lines containing other adrenergic receptor subtypes was poor. The correlation coefficients for alpha-2B (neonatal rat lung), alpha-2C (OK cell), and alpha-2D (bovine pineal gland) were 0. 15, 0.68, and 0.81, respectively. These data suggest that the predominant alpha-2 adrenergic subtype present in human spinal cord is the alpha-2A subtype. Both [H-3]rauwolscine and [H-3]RX821002 appeared to label a single class of binding sites. The alpha-2 adrenergic receptor density was significantly greater in the sacral region of the cord as compared to either the lumbar or thoracic regions.