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1 Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN, USA
* To whom correspondence should be addressed. E-mail: charkoudian.nisha{at}mayo.edu.
Plasma osmolality has been shown to alter control of sympathetic activity and heart rate in animal models; however, it is unknown whether physiological increases in plasma osmolality have such influences in humans, and what effect concurrent changes in central venous and/or arterial pressures may have. We tested whether physiological increases in plasma osmolality (similar to those seen during exercise dehydration) alter control of muscle sympathetic nerve activity (MSNA) and heart rate (HR) in humans. We studied 17 healthy young adults (7F, 10M) at baseline, and during arterial pressure transients induced by sequential injections of nitroprusside and phenylephrine, in each of three conditions: control (C), after 1 ml/kg intravenous hypertonic saline (HT1) and after 2 ml/kg hypertonic saline (HT2). We continuously measured HR, arterial pressure, central venous pressure (CVP; peripherally inserted central catheter) and MSNA (peroneal microneurography) in all three conditions. Plasma osmolality increased from 287 ± 1 mOsm/kg in C to 290 ± 1 mOsm/kg in HT1 (P < 0.05), but did not increase further in HT2 (291 ± 1 mOsm/kg; P > 0.05 vs. C). Mean arterial pressure and CVP were similar between C and HT1, but both variables increased slightly in HT2. HR increased slightly but significantly during both HT1 and HT2 compared to C (P < 0.05). In HT1, the sensitivity of baroreflex control of MSNA was significantly increased compared to C, (-7.59 ± 0.97 [HT1] vs. -5.85 ± 0.63 [C] au/beat/mmHg, P < 0.01) but was not different from C in HT2 (-6.55 ± 0.94 au/beat/mmHg). We conclude that physiological changes in plasma osmolality significantly alter control of both MSNA and HR in humans, and that this influence can be modified by CVP and arterial pressure.
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