COUNTER-REGULATORY RESPONSES TO CONTINUOUS AND INTERMITTENT THERAPY WITH NITROGLYCERIN

Background. Vasodilatory therapy may be associated with reflex counter-regulatory responses, and these responses may play a role in the development of tolerance to nitroglycerin (GTN). Methods and Results. Standing systolic blood pressure, body weight, urinary sodium, and hormonal responses to continuous (n = 10) and intermittent (n = 10) transdermal GTN administration were studied in normal volunteers. There was rapid attenuation of the hypotensive response to transdermal GTN therapy in the continuous but not in the intermittent therapy group. Significant weight gain and sodium retention occurred during continuous but not during intermittent GTN therapy. This was accompanied by a greater decrease in hematocrit in the continuous group, a finding that suggests that plasma volume expansion occurred during continuous GTN therapy. Continuous GTN therapy was associated with increases in plasma norepinephrine, atrial natriuretic peptide, arginine, vasopressin, and plasma renin activity. A different pattern of neurohormonal response was seen during intermittent therapy, with values tending to return to baseline levels after the nitrate-free interval. Conclusions. Continuous transdermal GTN therapy leads to counter-regulatory responses associated with sodium retention and probable plasma volume expansion. By contrast, intermittent transdermal GTN therapy is associated with a different pattern of hormonal response, the lack of sodium retention and no evidence of plasma volume expansion. It is likely that these counter-regulatory responses play an important role in the attenuation of nitrate effects.