Mg2+-Malate Co-Transport, a Mechanism for Na+-Independent Mg2+Transport in Neurons of the LeechHirudo medicinalis

Mg2+-extrusion from Mg2+-loaded neurons of the leech, Hirudo medicinalis, is mediated mainly by Na+/Mg2+antiport. However, in a number of leech neurons, Mg2+is extruded in the nominal absence of extracellular Na+, indicating the existence of an additional, Na+-independent Mg2+transport mechanism. This mechanism was investigated using electrophysiological and microfluorimetrical techniques. The rate of Na+-independent Mg2+extrusion from Mg2+-loaded leech neurons was found to be independent of extracellular Ca2+, K+, NO3−, HCO3−, SO42−, HPO42−, and of intra- and extracellular pH. Na+-independent Mg2+extrusion was not inhibited by 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), furosemide, ouabain, vanadate, iodoacetate, 4-amino-hippurate, or α-cyano-4-hydroxycinnamate and was not influenced by changes in the membrane potential in voltage-clamp experiments. Na+-independent Mg2+extrusion was, however, inhibited by the application of 2 mM probenecid, a blocker of organic anion transporters, suggesting that Mg2+might be co-transported with organic anions. Extracellularly, of all organic anions tested (malate, citrate, lactate, α-ketoglutarate, and 4-amino-hippurate) only high, but physiological, concentrations of malate (30 mM) had a significant inhibitory effect on Na+-independent Mg2+extrusion. Intracellularly, iontophoretically injected malate, citrate, or fura-2, but not Cl−, α-ketoglutarate, glutamate, succinate, or urate, were stimulating Na+-independent Mg2+extrusion from those neurons that initially did not extrude Mg2+in Na+-free solutions. Our data indicate that Mg2+is co-transported with organic anions, preferably with malate, the predominant extracellular anion in the leech. The proposed model implies that, under experimental conditions, malate drives Mg2+extrusion, whereas under physiological conditions, malate is actively taken up, driven by Mg2+, so that malate can be metabolized.

Anaerobic metabolism in the leech (Hirudo medicinalis L.): direct and indirect calorimetry during severe hypoxia

Anaerobic metabolism in the limnic annelid Hirudo medicinalis L. was investigated by direct and indirect calorimetry. During long-term severe hypoxia, the rate of heat dissipation was reduced up to 13% of the aerobic rate. At the same time, the rate of ATP turnover was reduced to about 30% of the aerobic rate, indicating that metabolic depression is an important mechanism to ensure survival of the leech during environmental anaerobiosis. Heat dissipation during hypoxia was monitored under two experimental conditions, favouring either concomitant hypocapnia (continuous N2 bubbling) or hypercapnia (self-induced hypoxia). The reduction in heat dissipation during hypocapnic hypoxia was less pronounced than during hypercapnic hypoxia, indicating that the different experimental conditions may influence anaerobic metabolism and the extent of metabolic depression. Biochemical analysis of known anaerobic substrates and endproducts provided the basis for indirect calorimetry during self-induced hypoxia. From changes in metabolites, the expected heat dissipation was calculated for initial (0-8 ,h) and long-term severe hypoxia (8-72 h). During the initial period, the calculated heat dissipation fully accounted for direct calorimetric determination. During long-term hypoxia, only 71% of the measured heat production could be explained from biochemical analysis of metabolites. Therefore, an additional unknown endproduct cannot be excluded, especially when anaerobic ammonia production and analysis of the carbohydrate balance are considered.

Hyperosmotic acclimation in the leech, Hirudo medicinalis L.: energy metabolism, osmotic, ionic and volume regulation

1. Acclimation of Hirudo medicinalis L. to hyperosmotic medium (12/1000 salinity) is characterized by changes in energy metabolism. A transient succinate fermentation is followed by malate accumulation in the tissue from the control level of 41.4 +/- 7.6 to 82.8 +/- 15.4 mumol/g dry weight after 9 days. 2. Initial osmotic water loss of 40% is restored in the intravasal space by isosmotic volume regulation after 9 days, but restoration is incomplete in the tissue even after 6 weeks of acclimation. 3. Elevation of SCCA anions in the blood is a mechanism of chloride hyporegulation.

Recovery after anaerobic metabolism in the leech (Hirudo medicinalis L.)

Medicinal leeches (Hirudo medicinalis L.) responded to self-induced hypoxia (72 h) with typical anaerobic metabolism characterized by a decrease in adenylate energy charge, utilization of the substrates glycogen and malate, and accumulation of the main anaerobic end-products succinate and propionate. Propionate was also excreted into the medium. Ammonia excretion was suppressed. Aerobic recovery resulted in a profound O2 debt. Resynthesis of ATP was completed within 30 min. Disposal of succinate and restoring of malate required 2-3 h, and clearance of propionate and recharging of glycogen 6-12 h. Ammonia excretion did not exceed normoxic rates and excretion of propionate during recovery accounted for only 10% of total propionate accumulated during hypoxia. It is postulated that the clearance of succinate and propionate involves oxidation but also resynthesis of malate and glycogen. During hypoxia and recovery blood osmolality remained constant. The Na+ and Cl- ion concentrations in blood, the decrease of which was nearly equimolar during hypoxia, were re-established following different time-courses. Na+ concentration returned to normoxic levels after 2-3 h. The delayed increase in Cl- concentration, however, correlating with 6-12 h necessary to clear blood propionate, is interpreted as an anion regulating effect.

Osmotic and ionic regulation during hypoxia in the medicinal leech, Hirudo medicinalis L

The concentrations of inorganic and organic ions and osmolality in the blood of the medicinal leech, Hirudo medicinalis, were determined during normoxia and hypercapnic and hypocapnic hypoxia. In normoxic animals, the blood sodium concentration was 124.5 +/- 4.2 mmol/l and the total cation concentration was 132.2 +/- 4.3 mEq/l (mean +/- S.D.). Major anionic compounds were chloride (40.8 +/- 1.6 mmol/l), bicarbonate (8.4 +/- 1.3 mmol/l), and organic anions (42.5 +/- 2.3 mEq/l). Among the latter, malate accounts for 30.4 +/- 2.2 mEq/l. The nature of the remaining anion fraction, which balances cation and anion concentrations in leech blood, remains unknown. Within 96 h of hypercapnic hypoxia, the amount of organic osmolytes in leech tissue increased from the control level of 56.6 +/- 9.1 to 158.3 +/- 19.5 mumol/g dry weight. An even higher amount of organic acids was accumulated within 96 h of hypocapnic hypoxia (218.0 +/- 53.7 mumol/g dry weight). A possible reason for this is that lactate, which is a major end-product of hypocapnic hypoxia, cannot be excreted to the external medium as easily as propionate. The accumulation of blood organic acids generating osmotic stress in the animals was compensated by an equimolar decrease in sodium and chloride ion concentrations. In hypercapnic animals these changes resulted in a constant osmotic concentration of the blood (200 mosmol/kg H2O) during the experimental period. Between 24 and 96 h of hypocapnic hypoxia, however, the increase in the osmotic gradient between animal and medium was correlated with further net water uptake and the obvious deterioration of the volume- and ion-regulatory mechanisms in these animals.