Effects of imipramine of the orthostatic changes in blood pressure, heart rate and plasma catecholamines

The effect of imipramine on the orthostatic changes in heart rate, blood pressure and plasma catecholamines were examined in six healthy male subjects on two occasions on high sodium balance (Na+ excretion greater than 120 mmol per day) and on low sodium balance (Na+ excretion less than 110 mmol per day), respectively. Orthostatic tests were carried out before and 2 h after ingestion of 150 mg imipramine hydrochloride. Imipramine caused a moderate increase in supine systolic blood pressure, and a pronounced increase in the rise in heart rate, when the subjects assumed erect position. The orthostatic drop in systolic blood pressure was in most cases only moderately increased after ingestion of imipramine, but in three subjects pronounced orthostatic hypotension developed when the sodium balance was low, whereas no clinical symptoms were seen in the same subjects when tested after imipramine ingestion on a high sodium balance. The plasma catecholamine levels in supine and standing position were not influenced by imipramine or by the changes in sodium balance. The data may suggest that inhibition of presynaptic reuptake of noradrenaline and/or alpha 2-adrenoceptor blockade causes the moderate rise in supine blood pressure, whereas alpha 1-adrenoceptor blockade, mainly affecting the venous part of the vascular bed, may explain the orthostatic reactions.