1. In order to define the alteration of the function of the adrenergic system in hypertension, we studied directly the receptor-cyclase coupling protein (N protein), which is one of the components of the enzyme adenylate cyclase.

2. N protein was determined in erythrocyte membranes of patients with essential hypertension and normal subjects, with a complementation assay in vitro. Fifteen normal subjects and 18 patients with essential hypertension (eight untreated and ten treated with β-adrenoreceptor blocking drugs alone or in combination with other antihypertensive drugs), and two patients with pseudohypoparathyroidism type Ia (known to have deficient N protein activity), were studied.

3. Erythrocyte N protein activities in the various groups expressed as percentages of the means ± sd of normals were: normal subjects 100 ± 13.7, untreated hypertensive 108.9 ± 20.4, treated hypertensive 104.3 ± 11.3 and pseudohypoparathyroidism type Ia 43%.

4. The difference between N protein activity in the hypertensive patients and normals was not statistically significant. We suggest that the molecular basis for the altered sympathetic responsiveness in essential hypertension may reside in other components of the cyclic AMP protein kinase effector system.

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