a-adrenergic receptor responsiveness and baroreflex control of muscle blood flow:human and animal studies

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[Purpose]:To assess the effects of a-adrenergic responsiveness in the vascular smooth mum/a on baroreflex control of muscle blood flow during exercise in animal and human studies.[Human studies]:1) Establishment of measurements:Determination of muscular vascular conductance: We determined the a-adrenergic responsiveness from change in forearm vascular conductance in response to such a small amount of phenylephrine injected into the brachial artery that systemic arterial pressure did not increase.Determination of baroreflex sensitivity: We determined baroreflex sensitivity from change in heart rate or muscle sympathetic nervous activity in response to spontaneous change in arterial pressure and also from change in heart rate or musk:, sympathetic nervous activity in response to a bolus injection of phenylephrine or sodium nitroprusside into the median vein.Application of doe measurements to PUTS (postural tachycardia syndrome) patients:We Sound in POTS patients that cardiac stroke volum … More e was lower during rest and exercise than in control subjects while peripheral a-adrenergic responsiveness was preserved.Effects of endurance training:We found that peripheral a-adrenergic responsiveness decreased and baroreflex sensitivity increased after endurance training in young healthy men.[Mouse studies]:We assessed baroreflex sensitivity response to acute hypovolemia in a mouse deficient of vasopressin V1a receptor and bind that cardiac baroreflex sensitivity decreased in the mouse while that increased in a control mouse.[Conclusions]These results suggest that peripheral a-adrenergic responsiveness decreased after endurance training which was accompanied by compensatory adaptation of an increase in baroreflex sensitivity. Vasopressin V1a receptors might be involved in this mechanism. However since we found that the reduced cardiac response to an upright posture in pars patients did not evoke an increase in baroreflex sensitivity, the enhanced baroreflex sensitivity might lie specifically caused by reduced a-adrenergic responsiveness in the peripheral vessels. Less