Plasma osmolality, urine composition and tissue water content of the toad Bufo viridis Laur. in nature and under controlled laboratory conditions

The compositions of plasma and urine were studied in toads (Bufo viridis) which were collected from three locations in Israel, and compared with toads which were kept under constant laboratory conditions for nearly 2 years. Plasma osmolality was rather constant (over 310 mOsm kg-1 H2O) during the whole year in the active toads. Urea was the most variable osmolyte in the plasma, and accounted for the higher osmolality in southern population. Urine osmolality fluctuated in a circannual fashion both in freshly captured and in the toads under constant laboratory conditions. Water content of the tissues was constant throughout the year, independent of the plasma osmolality. It is concluded that high plasma urea concentration and the excretory system (kidneys and the urinary bladder) are important in sustaining constant plasma osmolality in active toads. Both mechanisms change annually and form the basis for the high terrestriality of this species.