| Abstrakt: |
To determine the effects of alpha-melanocyte stimulating hormone (αMSH) on the circulation of undisturbed fetal sheep and young lambs and to explore possible mechanisms of actions, we examined the responses of ascending aortic blood flow, heart rate, stroke volume, and arterial blood pressure to a single intravenous injection of αMSH, ACTH, cortisol and to αMSH after beta-adrenergic blockade. We also measured cardiac output and organ blood flows before and after αMSH injection by the radionuclide labeled microsphere technique. αMSH increased ascending aortic blood flow from 249 ± 23 to 327 ± 30 ml ± min-1kg-1(P< 0.001), heart rate from 111 ± 10 to 126 ± 11 min-1, (P< 0.001) and stroke volume from 2.32 ± 0.21 to 2.75 ± 0.20 ml ± kg-1, (P< 0.001) in 10 young lambs 15–30-day-old. αMSH decreased mean systemic arterial blood pressure from 42.6 ± 1.7 to 39.3 ± 1.6 mmHg (P< 0.001) in fetal sheep and min-1, (P< 0.001) and stroke volume from 2.32 ± 0.21 to 2.75 ± 0.20 ml ± kg-1, (P< 0.001) in 10 young lambs 15–30-days-old. αMSH decreased mean systemic arterial blood pressure from 42.6 ± 1.7 to 39.3 ± 1.6 mmHg (P< 0.001) in fetal sheep and from 77.5 ± 4.5 to 72.7 ± 4.3 mmHg (P< 0.001) in young lambs. In six additional young lambs (20–35-day-old) cardiac output, measured by microspheres, increased with αMSH from 216 ± 20 to 261 ± 19 ml ± min-1± kg-1(P< 0.001). αMSH also increased blood flow to the myocardium from 104 ± 22 to 151 ± 26 ml ± min-1± 100 g-1(P< 0.001), to the adrenals from 195 ± 38 to 243 ± 41 ml ± min-1100 g-1, (P< 0.05), and to the lungs from 157 ± 31 to 468 ± 81 ml ± min-1± 100 g-1(P< 0.005). The increase in lung flow suggests an increase in peripheral arteriovenous shunting in young lambs. There was no change in blood flow to the brain, thyroid, spleen, kidneys, gut, thymus, liver, and carcass. ACTH or cortisol did not have cardiovascular effects in fetal or young lambs. Beta-adrenergic blockade with propranolol did not prevent the increase in aortic blood flow (from 296 ± 53 to 426 ± 75 ml ± min-1± kg-1, P< 0.001) and heart rate (from 81 ± 4 to 106 ± 3 min-1, P< 0.001) observed in four young lambs, 15–21 days of age, after the injection of αMSH, but prevented the fall in arterial blood pressure. These responses indicate that αMSH does not act through beta-adrenergic receptors of the heart of young lambs. The observation that the cardiovascular system of fetal and young lambs responds to αMSH suggests that αMSH or related substances could play a role in regulating the circulation in the perinatal period. |
