Susan W. Nicolson

The ionic basis of fluid secretion in insect Malpighian tubules: Advances in the last ten years

Abstract Current information on the ionic basis of fluid secretion in Malpighian tubules is reviewed, with the emphasis on electrophysiological data, almost all of which has appeared in the last decade. Different electrical techniques are compared. Evidence for the various channels, carriers and pumps thought to be involved in the transport of K, Na, H and Cl ions is discussed. Few insect species have been studied in any detail, and major differences between them are apparent. Rapid advances are anticipated when synthetic diuretic peptides become available for electrophysiological studies.

Water balance and osmoregulation in Onymacris plana, a tenebrionid beetle from the Namib desert

Onymacris plana, a tenebrionid beetle from the sand dunes of the Namib desert, lost weight very slowly during 12 days of dehydration at 26°C. Measurement of total lipid showed a gradual decline, the metabolic water produced being sufficient to maintain a constant water content. At the same time the haemolymph volume decreased by 66%. When given water the dehydrated beetles drank rapidly and their weight and haemolymph volume were restored to normal. Haemolymph osmolarity was closely regulated despite the changes in volume. Haemolymph potassium was also well regulated, but sodium was lost from the haemolymph during a cycle of dehydration and rehydration, even though sodium losses in the faeces were small. Water balance in Onymacris depends on efficient conservation of water in periods of drought and on water uptake by drinking during the coastal fogs of the Namib.

Diuresis or clearance: Is there a physiological role for the “diuretic hormone” of the desert beetle Onymacris?

Abstract Malpighian tubules of Namib Desert tenebrionid beetles of the genus Onymacris are strongly stimulated by homogenates of the corpora cardiaca. The corpora cardiaca of other arid-adapted tenebrionids also contain diuretic material. Biogenic amines, which could be released during the preparation of corpora cardiaca extracts, do not stimulate fluid secretion in tubules of Onymacris rugatipennis . The diuretic factor in corpora cardiaca extracts is stable to boiling and to incubation with pronase. HPLC separation of the corpora cardiaca of O. rugatipennis gives a single region with diuretic activity in both secretory and electrical bioassays. Diuretic activity can not be detected in the haemolymph of Onymacris , and injection of corpora cardiaca extracts into the beetles does not cause diuresis. Simultaneous injection of corpora cardiaca and the dye amaranth shows that the most of the dye transported by the Malpighian tubules moves anteriorly into the midgut, indicating fluid recycling by this route. The most likely function for this “diuretic hormone” is clearance of metabolic wastes from the haemolymph.

Pollen digestion by flower-feeding Scarabaeidae: protea beetles (Cetoniini) and monkey beetles (Hopliini).

Pollen protoplasm is very nutritious, but the hard and highly resistant outer wall (exine) of the pollen grain presents an obstacle that pollen-feeders must overcome to benefit from the valuable protoplasm. Pollen digestion in three pollen-eating scarab beetles, the green protea beetle Trichostetha fascicularis and two monkey beetle species, Peritrichia cinerea and Pachynema flavolineata, was investigated. Adult beetles were collected for observations of feeding mechanisms, scanning electron microscopy of mouthparts, and histological examination of gut contents. Serial sections of different regions along the gut were stained for different nutrients and examined to determine the appearance of the pollen grains and the removal of carbohydrates, proteins and lipids from the pollen. During feeding, beetles used dense brushes of setae on the tips of their maxillae to mop up nectar and sweep pollen into their mouths. Grains were ingested intact. Nutrients were effectively removed from the grains as they passed along the gut and most pollen grains in the hindgut were empty, with broken exines. All the beetles excreted the undigested exine. Comparison with other studies on pollen digestion by a variety of insect and vertebrate pollen-feeders suggests that digestive enzyme penetration of ingested pollen grains and exudation of the products of digestion is the most likely method used by these beetles.

Diuresis in a desert beetle? hormonal control of the malpighian tubules ofOnymacris plana (Coleoptera: Tenebrionidae)

SummaryMalpighian tubules of a desert tenebrionid beetle,Onymacris plana, have been studied as isolated preparations. Under control conditions tubules of female beetles secreted fluid at an average rate of 3.3 nl/min, but this rate was increased 20–25 times by a diuretic hormone (DH).Homogenates of the brain, corpora cardiaca (CC) and prothoracic ganglion induced striking increases in tubule secretion rates, which sometimes exceeded 100 nl/min. The increased rates were sustained for 3 h without renewal of the medium. Diuretic activity was also present in the other thoracic ganglia. High K treatment caused release of DH from the CC only.Exogenous cyclic AMP (1 mM) stimulated the isolated tubules ofO. plana, but to a lesser extent than the DH. The cationic composition of the secreted fluid resembled that of most other insect tubules, with high K and low Na concentrations. Stimulation with DH doubled the Na concentration.The DH was not inactivated by the tubules themselves, but was destroyed by contact with the haemolymph. An inactivation mechanism is vital in the apparently contradictory situation of a desert beetle possessing a diuretic hormone. The role of the cryptonephric system during diuresis is unknown.

Energy and water balance in the lesser double-collared sunbird (Nectarinia chalybea) feeding on different nectar concentrations

Abstract The water balance of nectarivores is tightly linked to their energy balance. When nectar is dilute, consumption of a large water excess is inevitable. We investigated energy and water balance in lesser double-collared sunbirds, Nectarinia chalybea (8u2009g), kept at 20u2009°C and fed different nectar concentrations (0.4, 0.8u2009M sucrose or 1.2u2009M sucrose). The mass of sucrose consumed, body mass, day-time mass gain and night-time mass loss were the same irrespective of diet, the birds compensating energetically for changes in sucrose concentration by drinking greater volumes of the more dilute solutions. Sunbirds consumed between 0.5 times and 1.8 times their body mass in preformed water per day, depending on sucrose concentration, and excreted around 75% of the water. The difference between water gain (preformed and metabolic water) and excreted water is assumed to equal evaporative water loss, and was similar on 1.2u2009M and 0.8u2009M sucrose, but was higher on a diet of 0.4u2009M sucrose. The osmolalities and K+ and Na+ concentrations of the excreted fluid were extremely low, so that sunbird urine resembled that of hummingbirds and freshwater vertebrates rather than that of typical terrestrial vertebrates. N. chalybea is able to maintain energy and water balance over a range of nectar concentrations by adjusting the volume of solution consumed and by excreting copious, dilute fluid.

Diuresis and its hormonal control in butterflies

Abstract The sudden weight loss after eclosion in the butterflies Acraea horta, Danaus chrysippus and Papilio demodocus is largely due to diuresis. The potassium concentration of the haemolymph is approximately halved as a result, and extensive diuresis leads to increases in sodium concentration and total osmolarity. The isolated Malpighian tubules of all three species are stimulated to fast rates of fluid secretion by cyclic AMP and by homogenates of the brains and corpora cardiaca. The tubules of Papilio lose their sensitivity to stimulation after the first day of adult life, and ingestion of a large volume of artificial nectar by this butterfly does not cause diuresis.

Osmoregulation in a nectar-feeding insect, the carpenter bee Xylocopa capitata: water excess and ion conservation

Abstract The liquid diet and high metabolic water production during flight in the carpenter bee Xylocopa capitata Smith 1854 (Hymenoptera, Anthophoridae) causes a water excess, and this is exacerbated by a low dietary intake of ions. The nectar and pollen of the preferred food‐plants, Virgilia divaricata Adamson and Podalyria calyptrata Willd., and other Fabaceae had low levels of sodium. Analyses of the bees and their body fluids showed that the bees have an exceptionally low Na content, and Na homeostasis seems to depend on recycling almost all Na which enters the rectum. The copious dilute urine (137 mOsm) had Na and K concentrations of only 3.4 and 7.0 ITIM, respectively. Isolated preparations of Xylocopa Malpighian tubules secreted a fluid with a K concentration 10 times that of the haemolymph. This means that recycling of K is as important as that of Na, and the bulk of K resorption probably occurs passively in the ileum. This study is the first to examine hymenopteran Malpighian tubules. Their stimulation by cAMP is indicative of the presence of a diuretic hormone in Xylocopa.

Pollen and the Nitrogen Requirements of the Lesser Double-collared Sunbird

Nectarivorous birds typically are small, often weighing less than 10 g, and have high basal metabolic rates. To meet their high energy needs, many nectarivorous species spend a large proportion of their time foraging on nectar. Like all birds, they also require nutrients such as protein, vitamins, essential fatty acids, minerals, and trace elements. Amino acids occur in floral nectar, and their concentrations are thought to be related to pollinator type, being low in nectar of bird-pollinated plants (Baker and Baker 1982). However, the correlation between amino acid concentration and pollinator type has been disputed by Gottsberger et al. (1984), who suggested that most of the variation in nectar amino acids results from flower damage or contamination with pollen. In any event, the amounts are too low to satisfy the protein requirements of nectarivorous birds (Martinez del Rio 1994). Other nitrogen sources are insects or pollen (Richardson and Wooller 1990, Brice 1992), both of which present birds with a number of potential problems. In insects, a large proportion of the nitrogen is in the form of chitin, a polymer that may be difficult for a small nectarivorous bird to digest. Although some seabirds are capable of producing chitinases (Jackson et al. 1992), no nectarivorous birds are known to do so. Accordingly, the major nitrogen sources obtained from insects are likely to be the proteins, peptides, and free amino acids found in their tissues. However, the amino acid composition of insects varies greatly and often is dominated by nonessential amino acids such as proline (Tomlin et al. 1993). As a result, insects are not always as good a source of protein as is widely believed. Furthermore, it may be energetically very expensive for a small nectarivorous bird to catch enough insects to meet its nitrogen requirements. Pollen has long been recognized as a major source of nitrogen for various insect pollinators, most notably honeybees (Apis mellifera; Schmidt and Buchmann 1985, Dobson and Peng 1997), and it has recently become clear that pollen also is an important element in the diet of vertebrate pollinators (Howell 1974, Richardson and Wooller 1990, Law 1992, van

Effects of biogenic amines and hormones on butterfly Malpighian tubules: Dopamine stimulates fluid secretion

Abstract Isolated Malpighian tubules of Papilio demodocus, the citrus swallowtail butterfly, were stimulated by biogenic amines as well as by cyclic AMP and the naturally occuring diuretic hormone. The greatest secretory response was obtained with 5-hydroxytryptamine, and smaller responses with dopamine and noradrenaline, but none of these amines could induce the maximal secretion rates obtained with cyclic AMP and diuretic hormone. Various other biogenic amines, hormones and pharmacological agents, including adrenaline, had no effect on Papilio tubules. The blocking agents cyproheptadine, phentolamine and propranolol did not alter the tubule response to biogenic amines.