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

Copyright © 2000 EDIZIONI MINERVA MEDICA

language: English

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


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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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