Hormonal control of renal functions in insects.

Insect Malpighian tubules secrete an isosmotic, KCl-rich primary urine containing low concentrations of most other blood solutes. Neuropeptide diuretic hormones (DH), possibly related to vasopressin, stimulate tubular fluid secretion by 2- to 200-fold in response to water loading, e.g., feeding. DH acts on tubules through cyclic AMP (cAMP) to stimulate salt transport without measurable change in osmotic permeability. Changes in composition of tubular secretion after stimulation and the possible control of DH release are discussed. Most of the water, ions, and metabolites in tubular secretion are normally reabsorbed by active mechanisms in the rectum, where the urine may finally become either hyposmotic or strongly hyperosmotic to the blood. A newly discovered neuropeptide, chloride transport-stimulating hormone, controls (via cAMP) reabsorption of the principal salt by stimulating K-dependent, electrogenic transport of Cl- across the apical cell border. Passive net absorption of K+ is thereby enhanced. Diuretic and antidiuretic factors may control osmotic permeability of the rectal wall and thereby influence the osmotic concentrations of the rectal absorbate and final urine. The increased recycling of a KCl-rich fluid through the Malpighian tubule-rectal system after feeding probably serves to clear the body of unwanted substances ingested with, and produced by, metabolism of the meal.