Modulation of luteinizing hormone immunoreactivity and bioactivity by dopamine but not norepinephrine in women

It has been suggested that changes in bioactive luteinizing hormone in women occur toward midcycle and may result in increased bioactive/immunoreactive luteinizing hormone ratios. To determine whether dopamine or norepinephrine modulate immunoreactive and bioactive luteinizing hormone secretion, 15 ovulatory women were studied in the mid to late follicular phase. Dopamine in two doses (0.5 μg/kg/min and 4 μg/kg/min) and norepinephrine, 0.1 μg/kg/min, were infused for 4 hours, and metoclopramide, 10 mg intravenously, was also given to determine whether dopamine receptor antagonism results in changes. Bioactive luteinizing hormone and the bioactive/immunoreactive luteinizing hormone ratio increased in women from the early to late follicular phase (p < 0.05). Both intravenous doses of dopamine resulted in significant decrements in immunoreactive luteinizing hormone (20 ± 2 and 20 ± 3%, p < 0.02) and bioactive luteinizing hormone (36.7% and 43.2%, p < 0.05). With dopamine there was also a significant decrease in the bioactive/immunoreactive luteinizing hormone ratio (p < 0.02). Intravenous norepinephrine, however, resulted in no changes in either immunoreactive or bioactive luteinizing hormone levels. Metoclopramide also did not change immunoreactive or bioactive luteinizing hormone levels. These data suggest that although endogenous dopaminergic blockade may not play a significant role in determining basal levels of luteinizing hormone, decreases in dopamine at the pituitary level may increase the bioactive/immunoreactive luteinizing hormone ratio. Norepinephrine does not appear to exert major changes in immunoreactive or bioactive luteinizing hormone.