Locust diuretic hormone-stimulated synthesis and excretion of cyclic-AMP: a novel Malpighian tubule bioassay

Identification of a potent antidiuretic factor acting on beetle Malpighian tubules

Beetles, like other insects, depend on diuretic and antidiuretic hormones to control water balance. We have isolated, using head extracts from the beetle Tenebrio molitor, a peptide that strongly inhibits fluid secretion by the Malpighian tubules of this insect. This antidiuretic factor (ADF) appears to elicit its effect via cGMP as a second messenger but does not stimulate NO production. It has primary structure: Val-Val-Asn-Thr-Pro-Gly-His-Ala-Val-Ser-Tyr-His-Val-Tyr-OH. The ADF inhibits tubule secretion with high potency: the EC(50) is around 10 fM. It bears no significant resemblance to other biologically active neuropeptides. To our knowledge this is the only endogenous insect ADF acting on Malpighian tubules to be sequenced, and the first coleopteran (beetle) antidiuretic factor fully characterized to date.

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 μmol 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 μmol 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.

Isolation and characterization of a diuretic peptide common to the house fly and stable fly

An identical CRF-related diuretic peptide (Musca-DP) was isolated and characterized from whole-body extracts of the house fly, Musca domestica, and stable fly, Stomoxys calcitrans. The peptide stimulates cyclic AMP production in Manduca sexta Malpighian tubules and increases the rate of fluid secretion by isolated Musca domestica tubules. The 44-residue peptide, with a mol.wt. of 5180, is amidated, and has the primary structure: NKPSLSIVNPLDVLRQRLLLEIARRQMKENTRQVELNRAILKNV-NH2. Musca-DP has a high percentage of sequence identity with other characterized CRF-related insect diuretic peptides.

Neurotransmitters and stimulation of fluid reabsorption in migratory locust rectal cells

Several biogenic amines enhance fluid reabsorption and the accumulation of cyclic adenosine-monophosphate (cAMP) in the rectum of the migratory locust but only 5-hydroxytryptamine (5-HT) acts in a dose-dependent manner at low concentrations (between 10(-8) and 5.10(-7) M). Cyclic AMP is a second messenger of 5-HT, and its actions on fluid reabsorption are calcium-dependent. Polymyxin B (a protein kinase C inhibitor) mimics the actions of 5-HT on fluid reabsorption and on calcium-dependent cAMP accumulation. This suggests the presence of other sources of calcium and a possible relationship between several transduction systems within different rectal cells. The second messenger system mediating the 5-HT antidiuretic message differs from those involved in the transduction of the known locust antidiuretic hormones.

Identification of a diuretic hormone of Locusta migratoria

We have isolated a peptide from brains and corpora cardiaca of Locusta migratoria which is immunologically related to the diuretic hormone of Manduca sexta. We determined its structure as a 46 amino acid linear peptide with 43-50% identity to the M. sexta hormone. Moreover, we showed that the new peptide functions as a diuretic hormone in L. migratoria, stimulating urine production by Malpighian tubules and elevating levels of cAMP in tubules.

Cyclic AMP in the Malpighian tubule fluid and in the urine of Rhodnius prolixus

We demonstrate the presence of cyclic adenosine monophosphate (cAMP) in the fluid secreted by isolated Malpighian tubules of Rhodnius prolixus. In addition, we show that fifth-instar R. prolixus excrete cAMP in the urine after a meal of human blood. Nonstimulated isolated Malpighian tubules secrete small amounts of cAMP that increase about 10-fold after the addition of 5-hydroxytryptamine (5-HT). 5-HT is known to mimic R. prolixus diuretic hormone. The present findings demonstrate that 5-HT also acts via cAMP. The presence of cAMP in the rectal sac of the insect could be of importance in the differentiation of Trypanosoma cruzi and in the cycle of Chagas disease.

Cyclic AMP mediation of the hormonal stimulation of 14C-acetate incorporation by Heliothis armigera pheromone glands in vitro

Evidence is presented for hormone-controlled adenosine 3',5'-cyclic monophosphate (cAMP)-mediated incorporation of 14C-acetate by in vitro pheromone gland preparations of Heliothis armigera. Basal intracellular cAMP concentrations of in vitro pheromone glands increase 28-fold in the presence of methanolic brain-complex extracts after 5 min of incubation. This intracellular cAMP increase shows a dose-dependent response. Studies on isolated pheromone gland preparations show that 8-bromo-cAMP added to the incubation media stimulates acetate incorporation thereby mimicking the hormonal response. This effect was duplicated by forskolin and isobutyl-methylxanthine. In addition, forskolin exhibits a dose-dependent stimulatory effect on the incorporation of 14C-acetate by such isolated in vitro pheromone glands. High-pressure liquid chromatographic analysis of the radioactive products, extracted by hexane from pheromone glands and media, revealed a stimulation by forskolin of a product exhibiting the same retention time as (Z)-11-hexadecenal, the main pheromone component of H. armigera. In addition, thin-layer chromatographic analysis revealed co-elution with (Z)-11-hexadecenal on stimulation by 8-bromo-cAMP.

Diuretic action and immunological cross-reactivity of corticotropin and locust diuretic hormone

A functional similarity and immunological cross-reactivity between adrenocorticotrophic hormone (ACTH) and a locust diuretic hormone (DH) is reported. The functional similarity is expressed in that ACTH mimics DH by stimulating fluid secretion and cyclic AMP (cAMP) secretion in locust Malpighian tubules. Desacetyl-alpha-melanocyte-stimulating hormone is active to a lesser degree but no other POMC-derived peptide tested was found to follow suit. Immunological cross-reactivity is shown by a positive response of HPLC-purified DH with a specific ACTH radioimmunoassay as well as a significant reduction in DH activity (fluid secretion) after incubations with ACTH antiserum. However, ACTH and DH are different peptides since they do not share common separation characteristics on HPLC and ACTH does not induce a high excess secretion of cAMP by the tubule cell as does DH.

The role of calcium in diuretic hormone action on locust Malpighian tubules

Fluid secretion by the Malpighian tubules (MTs) of Locusta is drastically reduced in the absence of extracellular calcium. Verapamil (10(-4) M) inhibits basal secretion, whereas the ionophore A23187 (10(-5) M) elevates the secretory rate. Cyclic guanosine monophosphate (cGMP) stimulates fluid secretion at a concentration of 10(-3) M. A factor extractable in methanol from the storage lobes of the corpora cardiaca stimulates increased guanylate cyclase activity in MTs, resulting in a 10-fold elevation in intracellular cGMP levels. Attempts to separate the factor stimulating guanylate cyclase by high-performance size-exclusion chromatography proved unsuccessful. Neither 5-hydroxytryptamine (5-HT) (10(-4) M) nor A23187 (10(-5) M) are able to elevate intracellular cGMP levels. Elevations of both intracellular cAMP and cGMP levels in response to diuretic hormone (DH) are potentiated in the absence of extracellular calcium. Consistent with these elevations are increases in the rates of fluid secretion by tubules deprived of extracellular calcium. It is concluded that, although calcium has an important role in the regulation of fluid secretion, its role in the mechanism of hormone-stimulated secretion may be modulatory rather than regulatory.