Osmotic characteristics of isolated human and canine pancreatic islets

Cryopreservation protocols for pancreatic islets may be optimized by modeling the changes in volume that occur during cryoprotectant addition/dilution and cooling/warming. Modeling may be facilitated if it can be demonstrated that isolated islets behave as ideal osmometers where the equilibrium volume is a linear function of the reciprocal of the osmolality of the extracellular medium (the Boyle van't Hoff relationship). The present study was performed to test the hypotheses that: (i) human and canine islets exhibit a linear osmotic response during exposure to hypo-, iso-, and hyperosmotic solutions and to determine the human and canine islets osmotically inactive fraction (Vbp); (ii) to determine if the ionic properties of a concentrated electrolyte solution have an effect on the osmotic response; and (iii) to determine if the osmotic response of pancreatic islets varies between species (human and canine). Islets were isolated from four human and four canine pancreases using standard collagenase digestion and EuroFicoll purification. After 12 h incubation at 37 degrees C, islets (n = 4 from each isolation) were exposed to varying salt or sucrose concentrations (75-1200 mOsm/kg). Islets were held by a micropipette system attached to an inverted microscope, and solutions were exchanged while videotaping. An imaging system was used to determine the cross-sectional area of the islet which was then used to estimate the spherical volume. The results of linear regression analysis indicated that both human and canine islets respond ideally to osmotic conditions within a range of 150-1200 mOsm/kg in both the electrolyte (salt) and the nonelectrolyte (sucrose) solutions. The Vbp value (mean +/- SD) of the four donors was 0.513 +/- 0.068 for canine islets in sucrose solutions, 0.474 +/- 0.068 for canine islets in salt solutions, 0. 425 +/- 0.073 for human islets in sucrose solutions, and 0.39 +/- 0. 073 for human islets in salt solutions. There was a significant difference between human and canine islets (P < 0.05) but not between experimental solutes (NaCl or sucrose) (P < 0.05). These data may be used in conjunction with osmotic tolerance data to develop improved methods for the cryopreservation of isolated islets.