ION AND FLUID SECRETION BY DIFFERENT SEGMENTS OF THE MALPIGHIAN TUBULES OF THE BLACK FIELD CRICKET TELEOGRYLLUS-OCEANICUS

Cricket Malpighian tubules have two morphologically distinct segments, a thin distal segment, which occupies approximately 10% of the total tubule length, and a main segment. The two segments differ in secretio n rates and response to corpora cardiaca extract. The secreted fluids differ in osmotic concentration and elemental composition. The distal segment secretes fluid at a rate (per mm length) which is approximatel y twice that of the main segment under control conditions. After stimu lation by corpora cardiaca extract (Cc) the rate from the main segment approximately doubles whilst the distal segment rate remains unchange d. Fluid from the main segment and the whole tubule is slightly hypo-o smotic to the medium (5-11 mosmol kg-1) under control conditions, wher eas that from the distal segment is slightly hyperosmotic (12 mosmol k g-1). On stimulation with Cc, the whole tubule fluid becomes slightly hyperosmotic (12 mosmol kg-1), that from the main segment remains slig htly hypo-osmotic (3 mosmol kg-1) but fluid from the distal segment be comes very hyperosmotic (55 mosmol kg-1). Differences between the tubu le fluid and the medium osmolality are indicated in parentheses. Fluid from the main segment has high concentrations of K (166 mmol l-1), Cl (111 mmol l-1), Na (41 mmol l-1) and P (83 mmol l-1), whereas that fr om the distal segment has high concentrations of K (101 mmol l-1) and Cl (137 mmol l-1). On stimulation with Cc, the elemental concentration s in fluids from the main segments and whole tubules do not change sig nificantly but the K and Cl concentrations in distal segment fluid inc rease (182 and 188 mmol l-1 respectively). The Mg present in whole tub ule fluid is derived largely from the distal segment. The ionic compos ition accounts for the observed osmotic concentrations in fluid from w hole tubules, main segments and stimulated distal segments, but not fo r the concentrations in fluid from unstimulated distal segments. The f luid from unstimulated distal segments contains an unidentified organi c solute accounting for approximately 90 mosmol kg-1 of the osmotic co ncentration. The distal segment contributes 22% and 11% of the fluid v olume, 26% Cl, 14% K and 12% Cl, 11% K in control and Cc-stimulated tu bules respectively. Considerably higher values are observed in individ ual tubules. The distal segment makes a significant contribution to th e total ion output of the tubule. The cyclic AMP content of tubule seg ments treated with corpora cardiaca extract was found to increase in b oth main and distal segments. When expressed in terms of protein conte nt there was no difference between segments. However. in terms of tota l cell volume, the cells of the distal segment had a tenfold greater c yclic AMP content than those of the main segment. This is consistent w ith a 10- to 20-fold higher secretion rate of K by the distal segment. It is suggested that the distal segment, whilst having a higher lengt h-specific fluid secretion rate than the main segment, is, nevertheles s. concerned primarily with ion and solute secretion since it is unres ponsive to diuretic hormone. The prime role of the main segment, which does respond to diuretic hormone. is fluid secretion. There appear to be major differences in hydraulic conductivity between the two segmen ts.