Secretion of primary urine by glomeruli of the hagfish kidney

Adrenaline and noradrenaline increased the perfusion pressure (Pperf) and single glomerulus filtration rate (SGFR) of perfused hagfish glomeruli. Small amounts (0.1 % or 0.5 %) of bovine serum albumin (BSA) in perfusion fluids containing Ficoll 70 did not diminish the loss of colloid from hagfish glomerular capillaries as has been reported for other perfused capillaries. However, replacement of Ficoll 70 with an osmotically equivalent amount (3 %) of BSA appreciably reduced colloid loss. It was concluded that adrenaline and colloids enhanced flow through the urine-forming capillaries. Whereas adrenaline elevated the SGFR, colloid lowered the SGFR probably by a direct effect on the fluid permeability of the capillary walls. The flow-enhancing effect of adrenaline was used to ensure the exposure of urine-forming capillaries to two inhibitors of active fluid transport, ouabain and 2,4-dinitrophenol (DNP). Both substances lowered the single glomerulus filtration fraction (SGFF), probably by affecting a fluid secretion mechanism. In addition, DNP diminished the flow-enhancing effect of adrenaline. This study provides relatively unequivocal evidence that fluid secretion underlies the formation of primary urine by the hagfish.

Stimulation of fluid secretion of malpighian tubules of drosophila melanogaster meig. by cyclic nucleotides of inosine, cytidine, thymidine and uridine

External application of the 3',5'-cyclic monophosphates of inosine, cytidine, uridine and thymidine stimulated the fluid secretion rate (FSR) of Malpighian tubules isolated from Drosophila melanogaster. The evidence suggested that the cyclic nucleotides acted intracellularly in some capacity. Receptors of the 'purinergic' type appeared not to be major contributors to fluid secretion; of three purinergic agonists tried, adenosine, adenosine 5'-monophosphate (AMP) and adenosine 5'-triphosphate (ATP), only adenosine had an effect, but this was not observed consistently. None of the purinergic agonists interfered with the stimulation of the FSR by adenosine 3',5'-cyclic monophosphate (cAMP). The maximum stimulation of the fluid-secretion rate by any cyclic nucleotide was approximately double the unstimulated (control) rate. Tubules stimulated to less than maximal FSR by one cyclic nucleotide could be stimulated maximally by an appropriate concentration of another cyclic nucleotide. Malpighian tubules bathed in solutions that contained either [3H]cAMP or [3H]cGMP accumulated radioactivity to a level many times that in the medium. Accumulation of radioactivity by tubules bathed in 430 nmol l-1 [3H]cAMP was suppressed by 1 mmol l-1 non-radioactive cyclic nucleotides in the order cAMP>>cGMP>cIMP>cCMP; neither cTMP nor cUMP suppressed the accumulation of [3H]cAMP. Approximately 35 % of the [3H]cAMP and 80 % of the [3H]cGMP that entered the Malpighian tubule cells was metabolised to compounds that were not identified. It was concluded that cyclic nucleotides enter the Malpighian tubule cells by at least one transport mechanism which is particularly sensitive to purine-based nucleotides.

Analysis of fluid dynamics in perfused glomeruli of the hagfish eptatretus stouti (Lockington)

The capillary tuft of glomeruli of the hagfish mesonephros contains both 'low'-pressure and 'high'-pressure glomerular vessels (LPGVs and HPGVs). The existence of the HPGV raised the possibility that pressure filtration could occur in the hagfish kidney when the blood pressure was sufficiently high. Therefore, measurements of glomerular capillary pressure were made in HPGVs and LPGVs whilst single glomeruli were perfused with hagfish Ringer's solution that contained the colloid Ficoll 70. Calculations of the effective colloid osmotic pressure in perfused capillaries were made; these showed that hydrostatic pressures within the HPGV were inadequate to effect pressure filtration except at high rates of perfusion. However, high rates of perfusion provoked perfusion pressures that exceeded the highest values measured in the renal blood supply of lightly anaesthetised hagfish. It was concluded that some process other than pressure filtration must account for formation of the primary urine by hagfish glomeruli. The proportion of the perfusate that became urine, the single glomerulus filtration fraction (SGFF), bore a strong positive relationship to the vascular resistance of perfused glomeruli. Both the SGFF and the vascular resistance were inversely related to the rate of perfusion except when that rate was very high. From these two observations it was concluded that at least two flow pathways exist in hagfish glomeruli: one that has a high vascular resistance and that contributes to the elaboration of the urine, and one that has a low vascular resistance and does not contribute to urine formation. The possible anatomical location of the various flow pathways through hagfish glomeruli and how they may function are discussed.