Decreased α₂-Adrenergic Receptors on Platelet Membranes from Diabetic Patients with Autonomic Neuropathy and Orthostatic Hypotension*

Platelet adrenergic receptors were studied in normal subjects and diabetic patients with autonomic neuropathy to determine the relationship between adrenoreceptor status and orthostatic hypotension. The binding of [3H]clonidine and [3H]yohimbine to platelet membranes was measured in diabetic patients with autonomic neuropathy and orthostatic hypotension (n = 12) and without orthostatic hypotension (n = 11), diabetic patients without autonomic neuropathy (n = 12), and normal subjects (n = 9). Mean basal and standing plasma norepinephrine levels were not different in the four groups, and there was no relationship between orthostasis and norepinephrine responses. The diabetic patients with orthostatic hypotension had a significantly greater fall in mean blood pressure [31 +/- 2.8 (+/- SE) mm Hg] than any of the other three groups. Diabetic patients with diabetic autonomic neuropathy and orthostatic hypotension had a 30-40% decrease in number of platelet alpha 2-adrenergic receptors, as demonstrated by [3H]clonidine and [3H]yohimbine binding. The maximum number of binding sites for clonidine was 34 +/- 2.8 (+/- SE) fmol/mg protein in normal subjects, 27.4 +/- 3.4 in diabetic patients with neuropathy, 26 +/- 2.5 in diabetic patients with autonomic neuropathy without orthostatic hypotension, and 20.4 +/- 3.8 fmol/mg protein in diabetic patients with autonomic neuropathy with orthostatic hypotension (P less than 0.001). The maximum number of binding sites for yohimbine was 112 +/- 12.6 in normal subjects, 127 +/- 10 in diabetic patients without orthostatic hypotension, and 87 +/- 12.4 fmol/mg protein in patients with diabetic autonomic neuropathy with orthostatic hypotension (P less than 0.001). Reduced platelet alpha 2-receptors are associated with postural hypotension in diabetic autonomic neuropathy. If applicable to the postjunctional alpha 2-adrenergic receptor on sympathetic neurons, reduced vascular responses to changes in posture would be expected despite normal or enhanced norepinephrine secretion.