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Background: No information is available on the effects of vasoactive drugs on the microcirculation and on the correlation between their effects on the capillary and the systemic circulation., Purpose: To characterize the effects of nitroglycerin (NTG), nitroprusside (NP), and metaraminol infusion on microcirculatory variables using a noninvasive monitoring system., Methods: Increasing doses of NTG (5-40 microg/kg/min) and NP (5-20 microg/kg/min) were intravenously infused to eight rabbits. The microcirculatory parameters were monitored by a combined noninvasive system which included laser Doppler flowmetry (LDF), photoplethysmograph (PPG), and transcutaneous oxygen tension (tc-PO2). Mean blood pressure (MBP) was measured simultaneously., Results: At maximal rate of NTG infusion, the values of LDF, PPG, tc-PO2, and MBP were 72.2 +/- 23.9, 234.7 +/- 165.6, 103.5 +/- 37.4, and 82.7 +/- 7.4% of baseline, respectively. Following NP infusion the LDF, PPG, tc-PO2, and MBP reached 70.8 +/- 24.1, 190.5 +/- 68.9, 70.0 +/- 14.7, and 75.6 +/- 14.6% of baseline, respectively. The tissue oxygenation was significantly reduced during NP infusion compared to NTG (P < 0.05). The values gradually returned to the baseline 10 min after termination of NP infusion, but not after NTG. After injection of 1 mg metaraminol there was a steep decline in LDF, PPG, and tc-PO2 parallel to a significant increase in MBP. When metaraminol was administered after injection of 0.1 mg phenoxybenzamine, these changes were abolished., Conclusions: This noninvasive system effectively monitors microcirculatory hemodynamic changes induced by vasoactive drugs. NTG and NP cause similar changes in the systemic and microcirculatory bed, with the exception of peripheral oxygenation, which is reduced only during infusion of NP. The changes in the microcirculatory bed following metaraminol and phenoxybenzamine imply that their effects on the arterioles are mediated by adrenergic alpha-receptors.