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The effect of intravenous salt loading on osmoregulation of hydrated glaucous-winged gulls, Larus glaucescens Raveendran, Lethika

Abstract

Renal function of fresh water acclimated Glaucous-winged Gulls, Larus qlaucescens, was studied during infusion of hypotonic and hypertonic NaCl. Two experimental protocols were followed. In one, the closed urine collection system (CCS), ureteral urine was collected using catheters glued over ureteral openings of a supine, previously anesthetized gull. In the other, the open urine collection system (OCS), ureteral urine was collected through a funnel placed in the urodeum of a standing, unanesthetized bird. In both protocols, there was continuous saline infusion of hypotonic (hydration) and hypertonic (LOAD) saline at 0.286 ml⋅min⁻¹. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), ml(kg⋅min) ⁻¹, were determined by ¹⁴C-polyethylene glycol (PEG) and ³H-para-aminohippuric acid (PAH) clearances. Plasma vasotocin (PAVT, pg⋅ml⁻¹) was measured. At the end of 4 h hydration with 0.02 M NaCl, urine flow was high but matched infusion rate only in CCS birds (CCS, 0.29 ± 0.05; OCS, 0.17 ± 0.03 ml⋅min⁻¹), GFR (CCS, 5.56 ± 0.85; OCS, 5.36 ± 0.77) and ERPF (CCS, 15.80 ± 1.60; OCS, 14.35 ± 1.65) were high; urine sodium (UNa+) concentration was low (CCS, 15.0 ± 7.3; OCS, 36.4 ± 6.0 mEq⋅1⁻¹), UNa+ excretion was low (CCS, 6.38 ± 4.2; OCS, 5.19 uEq⋅min⁻¹) ; urine/plasma PEG ratio (U/PPEG) was high (CCS, 22.4 ± 4.4, OCS, 39.6 ± 8.5); free water clearance (CH₂O) was positive (CCS, 0.143 ± 0.011; OCS, 0.052 ± 0.019 ml⋅min⁻¹) , and PAVT was low (ccs,14.7 ± 7.4; OCS, 16.1 ± 2.4) in both groups. Immediately following infusion of 5 M NaCl, GFR, ERPF and urine flow increased for about 10 mins. Fifteen minutes later, the GFR of CCS gulls fell to 70% of pre-load values (P < 0.05) and in OCS gulls, GFR and ERPF fell to 64% (P < 0.01) and 61% (P < 0.05). Eighty mins after infusion of 5 M NaCl, the GFR and ERPF of CCS gulls returned to pre-LOAD levels, but remained low in OCS gulls. Twenty-five minutes after salt load, urine flow had fallen to 49% (P < 0.05) and remained low. In OCS gulls, urine flow had fallen to 13% (P < 0.001) after 185 mins. In both CCS and OCS gulls, UNa+ concentration and excretion increased significantly. Sixty minutes after salt load, UNa+ excretion returned to pre-LOAD levels but UNa+ concentration remained high in CCS (111.7 ± 57.5) and OCS (132.8 ± 12.5) gulls. U/PPEG attained 134.3 ± 26.5 in CCS and 181.2 ± 32.4 in OCS gulls. CH₂O fell significantly (P < 0.05) in CCS gulls but remained unchanged in OCS gulls. Mean PAVT increased to 122.5 ± 5.5 in CCS and 96.0 ± 12.6 in OCS gulls. In both CCS and OCS gulls, salt gland secretion was initiated but ceased 60 mins after 5M NaCl infusion, although 60% of the load was retained in the gull.

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