Fluid secretion by isolated Malpighian tubules of Drosophila melanogaster Meig.: effects of organic anions, quinacrine and a diuretic factor found in the secreted fluid.

Para-aminohippuric acid (PAH, 0.2 and 1 mmol l(-)(1)) had no effect on the basal fluid secretion rate (FSR) of isolated Malpighian tubules of Drosophila melanogaster Meig. and did not affect stimulation of the FSR induced by adenosine 3',5'-monophosphate (cAMP). Phenol Red (phenolsulphonphthalein, PSP; 0.5 and 1 mmol l(-)(1)) slowed the FSR and abolished stimulation of the FSR by cAMP. Diodrast (1 mmol l(-)(1)) slightly, but significantly, reduced the FSR and greatly reduced the stimulation of the FSR normally provoked by cAMP and by the 3',5'-monophosphates of guanosine (cGMP), inosine (cIMP) and uridine (cUMP). However, stimulation of the FSR by the 3', 5'-monophosphate of cytidine (cCMP) was little affected by diodrast. Probenecid (0.2 or 1 mmol l(-)(1)) consistently stimulated the FSR, on average by approximately 25 %, but did not markedly inhibit the subsequent stimulation of the FSR by cAMP, cGMP or cIMP. However, the FSR of tubules stimulated by cGMP was temporarily lowered by probenecid. Quinacrine (0.1 mmol l(-)(1)) slowed basal FSR by an average of approximately 30 %, but subsequent stimulation of the FSR by cAMP was not noticeably affected. Both 0.1 mmol l(-)(1) cAMP and 1 mmol l(-)(1) probenecid stimulated adenylate cyclase activity in extracts of Malpighian tubules, but cIMP, cGMP, cUMP and diodrast were without effect in this regard. Uptake of radioactivity from a solution containing 500 nmol l(-)(1) [(3)H]cAMP and 9.5 micromol l(-)(1) cAMP was reduced by more than 90 % by 1 mmol l(-)(1) PSP, by approximately 40 % by 0.2 mmol l(-)(1) probenecid, by 36 % by 1 mmol l(-)(1) diodrast and by 30 % by 1 mmol l(-)(1) PAH. Neither 0.01 mmol l(-)(1) ouabain nor 0.1 mmol l(-)(1) quinacrine affected the uptake of [(3)H]cAMP by the Malpighian tubules. Fluid secreted by isolated Malpighian tubules of Drosophila melanogaster contains a factor that stimulated the FSR on average by approximately 50 %. The presence in the secreted fluid of cGMP at a concentration of 8.3 micromol l(-)(1) did not explain the stimulatory effect on FSR. These results support the existence of a carrier-mediated uptake of cyclic nucleotides into the Malpighian tubules of Drosophila melanogaster, possibly involving a multispecific transporter.

The absence of an arterial pressure effect on filtration by perfused glomeruli of the hagfish, Eptatretus stouti (Lockington).

Single renal corpuscles of hagfish were perfused with a Ringer solution containing Ficoll 70 to simulate the colloid osmotic pressure of hagfish plasma. Simultaneous measurements were made of single glomerulus filtration rate (SGFR), perfusion pressure and the pressure in a vessel of the renal vasculature. The results confirm that SGFR is independent of pressure in the glomerular capillaries (PGC). The results also suggest that flow through glomeruli and SGFR are closely linked. Studies of the pressures in glomerular capillaries during periods when the perfusion rate was varied indicate that PGC reflects the area of the active capillaries and the rate of perfusion. Therefore, in the hagfish, PGC appears to be an effect of factors that cause glomerular filtration, not the main cause of that process.

Hydrostatic pressures in glomeruli and renal vasculature of the hagfish, Eptatretus stouti.

Hydrostatic pressures in the renal vasculature of hagfish have been studied. Estimates of the blood colloid osmotic pressure (COP) have been made. In blood vessels supplying the renal corpuscles, the average hydrostatic pressure is about 1 kPa. The average hydrostatic pressure falls to a value of 0.04 kPa within the postcardinal vein efferent to the renal corpuscle. Within the glomerular capillaries the hydrostatic pressure averages 0.21 kPa. Since the blood COP averages about 1.4 kPa, it is clear that glomerular filtration in the hagfish is not underlain by the hydrostatic pressure of the arterial pulse. In some blood vessels efferent to the renal corpuscles, hydrostatic pressure may be as high as in the afferent supply. Evidence is presented that the glomerular capillaries are shunted by this high pressure vascular pathway.

Factors affecting glomerular function in the pacific hagfish Eptatretus stouti (Lockington).

Single glomerulus filtration rate in Eptatretus stouti averaged 20.3+/-2.13 (S.E.M.) nl min(-1). Single glomerulus glomerular filtration rate (GFR) could be correlated with arterial pressure when arterial pressure exceeded about 4 cm H2O. Glomerular filtration was affected by postglomerular resistance brought about by alteration of the volume of urinary spaces. Filtration undoubtedly plays a role in glomerulus function. However, average colloid osmotic pressure (COP) of the plasma is almost double the average hydrostatic pressure in the segmental arteries serving the glomeruli. The COP of glomerular fluid is essentially nil, therefore it is difficult to see how pressure filtration alone can account for primary urine formation. When single glomeruli were perfused with colloid-containing Ringer at pressures within the normal range of blood pressures, GFR was within the normal range. GFR was related inversely to the colloid osmotic pressure of the perfusion Ringer. Colloid entered the urine during perfusion. However, in only a few instances did this result in conditions favourable to pressure filtration. To assess the role of active processes in glomerular filtration, chemical inhibitors were added to the perfusion Ringer. Amiloride, acetazolamide, cyanide, 2-deoxy-D-glucose, iodoacetate and ethacrynic acid were without marked effect on glomerular filtration. Ouabain and dinitrophenol markedly reduced GFR; inhibition was probably not due to indirect effects upon the renal vasculature.

Ultrastructural tracer studies on the permeability of the Malpighian tubule of the pill millipede, Glomeris marginate (Villers).

The electron-dense tracers ferritin, and iron-dextran, and the protein horseradish peroxidase, have been used to investigate the ultrastructural basis of permeability in the upper and lower segments of the Malpighian tubules of Glomeris marginata. All these materials were able to cross the basal lamina and enter the tubule lumen of the upper segment, and it was established that horseradish peroxidase was able to enter the channels which interrupt the apical junctions. In the upper segment, ferritin, iron-dextran, and horseradish peroxidase are taken up and accumulated within intracellular vesicles. In the lower segment ferritin and iron-dextran enter the cells but become generally distributed over the cytoplasm, as well as entering membrane-bounded vacoules. The behaviour of horseradish peroxidase could not be assessed by owing to the presence of endogenous peroxidase activity in the cells. After fixation by direct application of glutaraldehyde to the undissected tubules, the extracellular spaces contained large numbers of membrane-bounded vesicles. The significance of these observations is discussed in relation to the physiological activities of the tubules.

Ultrastructural studies on the Malpighian tubule of the pill millipede, Glomeris marginata (villers).

Ultrastructural studies on the Malpighian tubules of Glomeris marginata (Villers) reveal considerable morphological differences between the upper, fluid secreting, segment, and the lower segment which is at present of unknown function. Previous reports have shown that the upper tubule has a high permeability to compounds of high molecular weight. This may be accounted for by the fact that the epithelium shows very extensive intercellular spaces which are linked directly to junctions apparently specialised to provide a low resistance extracellular pathway between the haemocoel and the tubule lumen. Histochemical studies on the localisation of phosphatase enzymes reveal intracellular vesicles with acid phosphatase activity. The basal labyrinth of the lower tubule exhibits considerable alkaline phosphatase activity which is apparently identical in location to the enzyme revealed by two different ATPase localisation techniques.