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Panminerva Medica 1999 June;41(2):119-24

lingua: Inglese

Tubular reab­sorp­tion and sodi­um excre­tion dur­ing urine rein­fu­sion

Romano G., Favret G., Federico E., Bartoli E.

From the Department of Internal Medicine, DPMSC University of Udine Medical School, Udine, Italy


Background. The mech­a­nisms respon­sible for the natriu­resis ­that fol­lows ­urine rein­fu­sion was inves­ti­gated in ­rats by clear­ance and micro­punc­ture tech­niques.
Methods. In ­each ­animal two ­urine rein­fu­sion ­periods (R1 and R2) ­were per­formed and com­pared to a non-­urine-rein­fu­sion, ­saline infu­sion ­period (S) sand­wiched ­between ­them.
Results. Switching ­from ­urine rein­fu­sion to an equiv­a­lent ­rate of ­saline ­loading was fol­lowed by a ­fall in Na excre­tion ­from 1.9±0.5 to 0.5±0.2% of fil­tered ­load, p<0.002. Urine osmo­lality ­rose, and ­urine to ­plasma ­inulin con­cen­tra­tion ­ratio ­rose sig­nif­i­cantly ­from 73±14 to 147±21 (p<0.002). The ­changes in GFR, ­SNGFR, abso­lute and per­cent prox­imal reab­sorp­tion ­could not ­account for ­these find­ings. A ­reduced Na excre­tion cou­pled to ­increased ­urine osmo­lality indi­cates ­enhanced trans­port ­along a seg­ment respon­sible for the uri­nary con­cen­trating mech­a­nism. Thus the ­data can be inter­preted ­then as due to ­enhanced reab­sorp­tion ­along the ­ascending ­limb of Henle’s ­loop. These ­changes ­were ­reversed by rein­sti­tuting ­urine rein­fu­sion ­after the S ­period. The con­sen­sual ­changes in Na+ and K+ excre­tion ­excluded an ­effect of ­urine rein­fu­sion on the ­distal ­exchange ­site. There was a con­tin­uous ­fall in prox­imal reab­sorp­tion ­from R1 (76±3%) to S (69±3%) to R2 (62±5%) ­which was ­inversely cor­re­lated ­with the ­changes in hem­a­tocrit (R = 0.49, p<0.026). This indi­cates ­that ­part of the ­late diu­resis and natriu­resis was due to ­volume expan­sion. An ­osmotic ­effect of rein­fused ­urine ­solutes was sug­gested by a ­late ­rise in ­plasma osmo­lality, ­from 312±13 to 323±8 mOsm/kg. Osmotic diu­resis ­could ­have ­exerted addi­tive ­effects ­upon ­those of ­volume expan­sion, ­accounting for the ­late ­fall in prox­imal reab­sorp­tion.
Conclusions. We con­clude ­that the ­acute ­effects of ­urine rein­fu­sion are due to ­changes in trans­port of ­solutes and perme­ability to ­water ­along ­distal ­tubular seg­ments. The ­changes in ­plasma osmo­lality ­during the ­last ­period of the ­present ­acute experi­ments, sug­gest the pos­sibility ­that ­solute reten­tion may be ­linked to the ­chronic ­effects of ­urine rein­fu­sion.

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