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WATER AND ELECTROLYTE HOMEOSTASIS

Swelling and pressure-volume relationships in the dermis measured by osmotic-stress technique

Plastic and Reconstructive Surgery Research, Wake-Forest University School of Medicine, Winston-Salem, North Carolina

Submitted 16 September 2008 ; accepted in final form 19 March 2009

Water transfer across the extracellular matrix (ECM) involves interstitial osmotic forces in as yet unclear ways. In particular, the traditional values of Starling forces cannot adequately explain fluid transfer rates. Here, we reassess these forces by analyzing fluid transfer in live pig and human dermal explants. Pressure potentials were controlled with inert polymers adjusted by membrane osmometry (range = 3–219 mmHg), and fluid transfer in and out of the explants was followed by sequential precision weighing. Water motional freedom in the dermis was examined by NMR. In pigs, mean hydration pressure (HP; the pressure at which volume did not change) was 107 ± 22 and 47 ± 12 (SE) mmHg at 4°C and 37°C (P = 0.012, paired t-test, n = 7). Volume changes observed in response to pressure potential were reversible. The equation, Volume change = V_max/[1+(time/T_1/2)^d], where V_max is maximal volume change; T_1/2, time at volume = 1/2 V_max; and d, a rate parameter, was fitted to experimental progression curves (r² > 0.9), yielding V_max values linearly related to pressure, with mean slopes –3.5 ± 0.28 and –2.6 ± 0.21(SE) µl·g^–1·mmHg^–1 at 4°C and 37°C. NMR spin-spin relaxation times (T₂) varied within 200- to 400-µm distances in directions perpendicular to the epidermis, with slopes reaching 0.03 ms/µm. Results support a mechanism in which fluid transport across the ECM is locally regulated at micrometer scales by cell- and fiber-gel-dependent osmomechanical forces. The large HP helps to explain the fast interstitial in/out flow rates observed clinically.

extracellular matrix; edema; skin interstitium; MRI