Letter: Adverse reactions to prazosin

Orthostatic hypotension occurs following α2-adrenoceptor blockade in chronic prazosin-pretreated conscious spontaneously hypertensive rats

1. Studies were performed to evaluate whether chronic prazosin treatment alters the alpha 2-adrenoceptor function for orthostatic control of arterial blood pressure in conscious spontaneously hypertensive rats (SHR). 2. Conscious SHR (male 300-350 g) were subjected to 90 degrees head-up tilts for 60 s following acute administration of prazosin (0.1 mg kg-1 i.p.) or rauwolscine (3 mg kg-1 i.v.). Orthostatic hypotension was determined by the average decrease (%) in mean arterial pressure (MAP femoral) over the 60-s tilt period. The basal MAP of conscious SHR was reduced to a similar extent by prazosin (-23%(-)-26% MAP) and rauwolscine (-16%(-)-33% MAP). However, the head-up tilt induced orthostatic hypotension in the SHR treated with prazosin (-16% MAP, n = 6), but not in the SHR treated with rauwolscine (less than +2% MAP, n = 6). 3. Conscious SHR were treated for 4 days with prazosin at 2 mg kg-1 day-1 i.p. for chronic alpha 1-adrenoceptor blockade. MAP in conscious SHR after chronic prazosin treatment was 14% lower than in the untreated SHR (n = 8). Head-up tilts in these rats did not produce orthostatic hypotension when performed either prior to or after acute dosing of prazosin (0.1 mg kg-1 i.p.). Conversely, administration of rauwolscine (3 mg kg-1 i.v.) in chronic prazosin treated SHR decreased the basal MAP by 12-31% (n = 4), and subsequent tilts induced further drops of MAP by 19-23% in these rats. 4. The pressor responses and bradycardia to the alpha 1-agonist cirazoline (0.6 and 2 micrograms kg-1 i.v.), the alpha 2-agonist Abbott-53693 (1 and 3 micrograms kg-1 i.v.), and noradrenaline (0.1 and 1.0 micrograms kg-1 i.v.) were determined in conscious SHR with and without chronic prazosin pretreatment. Both the pressor and bradycardia effects of cirazoline were abolished in chronic prazosin treated SHR (n = 4) as compared to the untreated SHR (n = 4). On the other hand, the pressor effects of Abbott-53693 were similar in both groups of SHR, but the accompanying bradycardia was greater in SHR with chronic prazosin treatment than without such treatment. Furthermore, the bradycardia that accompanied the noradrenaline-induced pressor effect in SHR was similar with and without chronic prazosin treatment despite a 47-71% reduction of the pressor effect in chronic alpha 1-receptor blocked SHR.(ABSTRACT TRUNCATED AT 400 WORDS)

The neurohumoral and hemodynamic response to orthostatic tilt in patients with congestive heart failure

Thirty-five patients with varying degrees of congestive heart failure were subjected to 60 degrees upright tilt. Eight of the patients with normal resting hemodynamics had elevated resting plasma norepinephrine levels (PNE) (p less than 0.001), but their response to upright tilt was similar to that in normal subjects: All had increases in heart rate, plasma norepinephrine (from 263 +/- 32 to 483 +/- 78 pg/mg, p less than 0.02) and plasma renin activity (from 4.8 +/- 0.9 to 13.7 +/- 7.6 ng/ml/hour, p less than 0.05). In 27 patients with high resting pulmonary wedge pressure and low cardiac index, resting PNE was higher (668 +/- 71 ng/ml), but PNE, plasma renin activity and heart rate did not increase significantly during tilt despite a fall in pulmonary capillary wedge pressure and cardiac index. In 18 of these patients, PNE rose during tilt, whereas in nine it did not change or fell; the resting hemodynamics and the hemodynamic response to tilt were not significantly different in these two groups. These data suggest that an abnormality of mechanoreceptor or baroreceptor function is common in patients with CHF. This abnormality corresponds in part to the severity of the resting hemodynamic abnormality, but among patients with severe CHF, the reflex neurohumoral abnormality may provide independent information about the severity of the disease.

Orthostatic hypotension after the first administration of prazosin in hypertensive patients: role of the plasma volume

1. Prazosin (2 mg, p.o.) was administered to nine patients with essential hypertension while intra-arterial pressure was recorded by an Oxford portable apparatus. In all patients, 30 min-3 h after the administration, systolic and diastolic pressure fell on assuming the upright posture and four patients fainted. No correlation was found between the degree of fall in pressure and the plasma concentration of the drug. Acute expansion of the plasma volume by means of 6% Dextran infusion reduced the orthostatic blood pressure fall in all cases and a significant inverse correlation was found between plasma volume and orthostatic fall of pressure. 2. After ten days of continuous treatment with prazosin, 2 mg daily, a significant decrease in blood pressure was observed while orthostatic hypotension disappeared, probably due to the plasma volume expansion induced by the drug.

The vasodilator--beta-blocker interaction--some determinants of its clinical success

A number of structurally dissimilar compounds, sharing the ability to induce vasodilatation have come into clinical usefulness as antihypertensive drugs. Their successful utilisation often depends critically on an appropriate combination with diuretics and beta-blockers. Beta-blockers can specifically inhibit the reflex cardiac stimulation which otherwise may limit the tolerability and haemodynamic value of these drugs. A synergistic antihypertensive effect has been demonstrated with hydrallazine and propranolol and possibly exists with other combinations. Generally, vasodilators are not first-line drugs in chronic therapy but are best added to beta-blockers. Variations of the dose-response and the time-course of effect are stressed as being important determinants of a successful interaction.