Effect of sulphonylurea on glucose-stimulated insulin secretion in healthy and non-insulin dependent diabetic subjects: a dose-response study

A microrheological examination of insulin-secreting β-cells in healthy and diabetic-like conditions

Pancreatic β-cells regulate glucose homeostasis through glucose-stimulated insulin secretion, which is hindered in type-2 diabetes. Transport of the insulin vesicles is expected to be affected by changes in the viscoelastic and transport properties of the cytoplasm. These are evaluated in situ through particle-tracking measurements using a rat insulinoma β-cell line. The use of inert probes assists in decoupling the material properties of the cytoplasm from the active transport through cellular processes. The effect of glucose-stimulated insulin secretion is examined, and the subsequent remodeling of the cytoskeleton, at constant effects of cell activity, is shown to result in reduced mobility of the tracer particles. Induction of diabetic-like conditions is identified to alter the mean-squared displacement of the passive particles in the cytoplasm and diminish its reaction to glucose stimulation.

Exercise, CaMKII, and type 2 diabetes

Individuals who exercise regularly are protected from type 2 diabetes and other metabolic syndromes, in part by enhanced gene transcription and induction of many signaling pathways crucial in correcting impaired metabolic pathways associated with a sedentary lifestyle. Exercise activates Calmodulin-dependent protein kinase (CaMK)II, resulting in increased mitochondrial oxidative capacity and glucose transport. CaMKII regulates many health beneficial cellular functions in individuals who exercise compared with those who do not exercise. The role of exercise in the regulation of carbohydrate, lipid metabolism, and insulin signaling pathways are explained at the onset. Followed by the role of exercise in the regulation of glucose transporter (GLUT)4 expression and mitochondrial biogenesis are explained. Next, the main functions of Calmodulin-dependent protein kinase and the mechanism to activate it are illustrated, finally, an overview of the role of CaMKII in regulating GLUT4 expression, mitochondrial biogenesis, and histone modification are discussed.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Assessment of pancreatic β-cell function: review of methods and clinical applications

Type 2 diabetes mellitus (T2DM) is characterized by a progressive failure of pancreatic β-cell function (BCF) with insulin resistance. Once insulin over-secretion can no longer compensate for the degree of insulin resistance, hyperglycemia becomes clinically significant and deterioration of residual β-cell reserve accelerates. This pathophysiology has important therapeutic implications. Ideally, therapy should address the underlying pathology and should be started early along the spectrum of decreasing glucose tolerance in order to prevent or slow β-cell failure and reverse insulin resistance. The development of an optimal treatment strategy for each patient requires accurate diagnostic tools for evaluating the underlying state of glucose tolerance. This review focuses on the most widely used methods for measuring BCF within the context of insulin resistance and includes examples of their use in prediabetes and T2DM, with an emphasis on the most recent therapeutic options (dipeptidyl peptidase-4 inhibitors and glucagon-like peptide-1 receptor agonists). Methods of BCF measurement include the homeostasis model assessment (HOMA); oral glucose tolerance tests, intravenous glucose tolerance tests (IVGTT), and meal tolerance tests; and the hyperglycemic clamp procedure. To provide a meaningful evaluation of BCF, it is necessary to interpret all observations within the context of insulin resistance. Therefore, this review also discusses methods utilized to quantitate insulin-dependent glucose metabolism, such as the IVGTT and the euglycemic-hyperinsulinemic clamp procedures. In addition, an example is presented of a mathematical modeling approach that can use data from BCF measurements to develop a better understanding of BCF behavior and the overall status of glucose tolerance.

Is beta-cell failure in type 2 diabetes mellitus reversible?

BACKGROUND:

In the UK Prospective Diabetes Study (UKPDS), many subjects maintained glycemic goal (HbA_1c < 7.0%) at 9 years, showing that β-cell function was preserved and that the initial decline in β-cell function recovered with sulphonylureas. Moreover, obese subjects using high daily doses of insulin for several years rarely require insulin or oral hypoglycemic agents to maintain their glycemic goal following weight loss achieved by gastric bypass surgery. Thus, declining β-cell function during the course of type 2 diabetes mellitus (T2DM) is neither universal nor permanent.

OBJECTIVE:

To assess β-cell function in morbidly obese subjects before insulin withdrawal and on attaining the glycemic goal with weight loss and oral agents.

MATERIALS AND METHODS:

Serum C-peptide (CPEP) and glucose (G) concentrations were determined up to 180 min during an oral glucose tolerance test (OGTT) with 75 glucose in 10 obese men with T2DM, before insulin withdrawal, and on achieving the glycemic goal with metformin, glimepiride, and weight loss. Ten age-matched healthy men participated as controls. Cumulative responses (CR) of CPEP and G were calculated by adding differences between the level at each time-period during OGTT and fasting (F) concentration. β-Cell function was expressed as the FCPEP as well as the insulinogenic index (CRCPEP/CRG). Insulin sensitivity was determined as FCEP × FG.

RESULTS:

FCPEP was decreased, though still present, prior to insulin withdrawal. Moreover, on attaining the glycemic goal over 6-9 months, FCPEP, CRPEP/CRG, and FCPEP × FG improved markedly (P < 0.001).

CONCLUSION:

Decline in β-cell function in morbidly obese T2DM may not be progressive and is reversible on improving insulin sensitivity and on eliminating the inhibition by exogenous insulin.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single β cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

Sulfonylurea treatment of type 2 diabetes mellitus: focus on glimepiride

Sulfonylureas, which have evolved through two generations since their introduction nearly 50 years ago, remain the most frequently prescribed oral agents for treatment of patients with type 2 diabetes mellitus. Glyburide, glipizide, and glimepiride, the newest sulfonylureas, are as effective at lowering plasma glucose concentrations as first-generation agents but are more potent, better tolerated, and associated with a lower risk of adverse effects. Differences in their binding affinity to the beta-cell sulfonylurea receptor have been described, with preservation of cardioprotective responses to ischemia with glimepiride. Clinical studies have shown glimepiride to be safe and effective in reducing fasting and postprandial glucose levels, as well as glycosylated hemoglobin concentrations, with dosages of 1-8 mg/day. In comparative trials, glimepiride was as effective in lowering glucose levels as glyburide and glipizide, but glimepiride was associated with a reduced likelihood of hypoglycemia and a smaller increase in fasting insulin and C-peptide levels than glyburide, and a more rapid lowering of fasting plasma glucose levels than glipizide. Glimepiride also improves first-phase insulin secretion, which plays an important role in reducing postprandial hyperglycemia. Insulin secretagogues, specifically glimepiride, merit consideration as first-line therapy for patients with type 2 diabetes.

Effects of glimepiride on insulin secretion and sensitivity in patients with recently diagnosed type 2 diabetes mellitus

BACKGROUND

The exact mechanism of the efficacy of glimepiride in the achievement of glycemic control has not yet been clearly defined.

OBJECTIVE

This study was conducted to examine the influence of glimepiride on insulin secretion and sensitivity in patients with type 2 diabetes mellitus (DM) of recent onset.

METHODS

This 24-week, open-label, controlled trial was conducted at the Division of Endocrinology and Metabolism, Veterans Affairs Medical Center (Phoenix, Arizona). Study participants were aged 32 to 75 years and had recent-onset (established by a short duration of symptoms 6 weeks to 6 months prior to the study) type 2 DM, or were age-matched healthy volunteers (control group). In the diabetic patients, glimepiride tablets were administered orally, initially at 2 mg once daily in the morning, with the dosage increased by 1 mg every 2 weeks until fasting plasma glucose (FPG) decreased to 6.7 mmol/L; the dosage was then maintained for the remainder of the 24-week study period. Oral glucose tolerance tests (OGTTs) were conducted in the control group and before treatment and at 24 weeks after the achievement and maintenance of glycemic control (glycosylated hemoglobin <7.0%) in the diabetic group. For OGTT, plasma insulin and glucose levels were determined after the subjects fasted overnight and then at every 15 minutes for 2 hours after glucose challenge.

RESULTS

Fourteen diabetic men (mean [SEM] age, 50 [6] years; range, 32-75 years) and 10 male healthy controls (mean [SD] age, 48 [5] years; range, 30-68 years) were enrolled. In the DM group, FPG decreased significantly after treatment ( P<0.001); fasting plasma insulin was markedly elevated before treatment (P<0.001 vs controls) and decreased after treatment ( P<0.01) but did not normalize; first-phase insulin secretion was markedly inhibited before treatment ( P<0.001 vs controls) and normalized after treatment ( P<0.001) total insulin secretion significantly improved after treatment ( P<0.01) but did not normalize. Finally, the pretreatment insulin sensitivity index decreased significantly (P<0.01) after treatment and normalized in 6 of 14 patients (42.9%) with type 2 DM.

CONCLUSIONS

In this study, glimepiride achieved desirable glycemic control in patients with recent-onset type 2 DM through improvement in insulin secretion and sensitivity.

Gliclazide at a lower concentration than therapeutic dose increases the sensitivity of insulin secretion to glucose in perfused rat pancreas

OBJECTIVES

We studied the difference between effects of therapeutic dose and sub-therapeutic dose of gliclazide on the glucose-induced insulin secretion.

METHODS

The normal rat pancreas was isolated and perfused with Krebs-Ringer buffer containing 1-14 mmol/l glucose. Influcences of 0.25 and 2.5 microg/ml gliclazide on the glucose concentration-insulin secretion curve was examined.

RESULTS

Gliclazide at 0.25 microg/ml significantly potentiated 5-8 mmol/l glucose-induced insulin secretion (2.5 +/- 0.5 vs 1.0 +/- 0.3 mU for 15 min at 6.5 mmol/l glucose, P<0.01), but did not give influence on either 1-3 or 10-14 mmol/l glucose-induced insulin secretion. The glucose concentration, at which half-maximal insulin secretion was observed, was lower with gliclazide (5.9 mmol/l) than in the control (7.5 mmol/l). Gliclazide at 2.5 microg/ml markedly increased the maximally glucose-stimulated insulin secretion from 3.9 +/- 0.5 mU for 15 min in the control to 6.6 +/- 0.7 mU for 15 min (P<0.01). The half-maximal insulin secretion was observed at a lower glucose concentration (5.0 mmol/l) than in the absence of gliclazide.

CONCLUSION

Gliclazide in sub-therapeutically low dose has different effects on insulin secretion from in therapeutic dose, namely sharpens the insulin secretion sensitivity to glucose with no influence on the maximal insulin secretion. It is possible that low doses of gliclazide might be of interest in some type 2 diabetics whose main pathophysiology is the blunting of insulin secretion response to hyperglycemia.

Glimepiride improves both first and second phases of insulin secretion in type 2 diabetes

OBJECTIVE

The purpose of this study was to assess the effect of glimepiride on insulin sensitivity and secretion in subjects with type 2 diabetes.

RESEARCH DESIGN AND METHODS

After a 2-week washout from prior sulfonylurea therapy, 11 obese subjects with type 2 diabetes underwent euglycemic and hyperglycemic clamp studies before and during glimepiride therapy.

RESULTS

Glimepiride resulted in a 2.4-mmol/l decrease in fasting plasma glucose (P = 0.04) that was correlated with reductions in postabsorptive endogenous glucose production (EGP) (16.4 +/- 0.6 vs. 13.5 +/- 0.5 micro mol. kg(-1). min(-1), P = 0.01) (r = 0.21, P = 0.01). Postabsorptive EGP on glimepiride was similar to that of control subjects (12.8 +/- 0.9 micro mol. kg(-1). min(-1), NS). Fasting plasma insulin (66 +/- 18 vs. 84 +/- 48 pmol/l, P = 0.05), and first-phase (19 +/- 8 vs. 32 +/- 11 pmol/l, P = 0.04) and second-phase incremental insulin responses to glucose (48 +/- 23 vs. 72 +/- 32 pmol/l, P = 0.02) improved with glimepiride therapy. Insulin sensitivity did not change with treatment (4.6 +/- 0.7 vs. 4.3 +/- 0.7 micro mol. kg(-1). min(-1). pmol(-1)) and remained below that of control subjects (8.1 +/- 1.8 micro mol. kg(-1). min(-1). pmol(-1), P = 0.04).

CONCLUSIONS

The current study demonstrates that glimepiride improves both first and second phases of insulin secretion, but not insulin sensitivity, in individuals with type 2 diabetes.

Effects of current therapeutic interventions on insulin resistance

Although diet and exercise remain the cornerstones of type 2 diabetes therapy, attempts at lifestyle changes seldom result in the achievement of glycaemic control. As a result, the addition of pharmacological agents is usually necessary. Currently available treatment options improve glycaemic control in the short term; however, maintaining long-term glycaemic control, halting disease progression, and preventing the complications of type 2 diabetes have all proven to be elusive therapeutic goals. For more than 30 years, sulphonylureas (SUs) have been first-line therapy for the management of type 2 diabetes. These compounds control hyperglycaemia by stimulating insulin release from pancreatic beta cells, and thus their benefits are limited to patients with preserved beta-cell function. Despite historic reliance on these agents to treat type 2 diabetes, long-term use of SUs may desensitize beta cells. The meglitinides (e.g. repaglinide) are a new class of non-sulphonylurea secretagogues that bind to a different receptor on the beta cell. Repaglinide has a short duration of action and may be useful for the treatment of postprandial hyperglycaemia. The biguanides (e.g. metformin) represent another class of antidiabetic agents and improve glycaemic control primarily by decreasing hepatic glucose output. Metformin and SUs provide similar glucose-lowering effects, and, in combination, may provide additional benefits in some patients. Reducing the rate of glucose absorption with alpha-glucosidase inhibitors (e.g. acarbose) has been explored as an alternative approach to the management of postprandial hyperglycaemia, but these agents do not address the primary defect in type 2 diabetes. Eventually, prolonged overproduction of insulin to compensate for hyperglycaemia leads to dramatically reduced beta-cell function, and exogenous insulin therapy is required.

Glucose-dependent insulinotropic effects of JTT-608, a novel antidiabetic compound

The effects of JTT-608 [trans-4-(4-methylcyclohexyl)-4-oxobutyric acid], a novel antidiabetic compound, on insulin secretion were investigated using mouse insulinoma cell line (MIN6 cells) and isolated, perfused rat pancreas. JTT-608 enhanced insulin secretion in MIN6 cells in a dose dependent (10-300 microM) and glucose concentration-dependent (2.8-16.7 mM) manner. Unlike sulphonylureas, JTT-608 minimally stimulated insulin secretion at low glucose concentrations but potently enhanced insulin secretion at high glucose concentrations. In isolated, perfused pancreas of normal rats, JTT-608 (100-300 microM) dose-dependently enhanced insulin secretion in the first and second phases at high glucose concentrations but minimally stimulated insulin secretion at a basal glucose concentration. In isolated, perfused pancreas of neonatally streptozotocin-induced non-insulin-dependent diabetes mellitus rats (nSTZ rats), JTT-608 (200 microM) normalized the first phase and doubled the second phase of insulin secretion. In MIN6 cells, JTT-608 did not inhibit the binding of [3H]glibenclamide to membrane fractions but enhanced K+-ATP channel-independent insulin secretion. These results suggest that JTT-608 enhances insulin secretion in a different manner and via a different mechanism from hypoglycemic sulphonylureas.

Sulfonylureas

Sulfonylureas have been available for the treatment of non-insulin-dependent diabetes mellitus (NIDDM) since the 1950s. With the introduction of new oral agents, there is a tendency to discount the value of sulfonylurea therapy. Sulfonylureas have the advantage of multiple formulations, low costs, minimal side effects, and demonstrated efficacy in controlling hyperglycemia. The major disadvantage of sulfonylureas is secondary failure, which may occur with all oral agents as part of the progression of NIDDM. Sulfonylureas should continue to play an important role in the treatment of NIDDM.

Hypoglycaemic and insulinotropic effects of a novel oral antidiabetic agent, (-)-N-(trans-4-isopropylcyclohexanecarbonyl)-D-phenylalanine (A-4166)

1. (-)-N-(trans-4-isopropylcyclohexanecarbonyl)-D-phenylalanine (A-4166), a novel oral hypoglycaemic agent is a non-sulphonylurea insulin secretagogue. 2. We investigated the insulin-releasing action and hypoglycaemic effect of A-4166 compared to sulphonylureas in vitro and in vivo. 3. A-4166 stimulated insulin secretion from rat freshly isolated pancreatic islets at concentrations from 3 x 10(-6) M to 3 x 10(-4) M in the presence of 2.8 mM glucose. There was no obvious difference in glucose dependency between the insulinotropic effect of A-4166 and that of glibenclamide, and no additive or synergistic effect was observed between these two drugs. 4. A-4166 displaced [3H]-glibenclamide bound to intact HIT-T15 cells in a concentration-dependent manner. The Ki value was 4.34 +/- 0.04 x 10(7) M, and the displacement potency of A-4166 was between that of glibenclamide and tolbutamide, being similar to that of gliclazide. 5. Inf fasted beagle dogs, A-4166 showed a dose-dependent hypoglycaemic effect after oral administration over the range 1 to 10 mg kg-1. The hypoglycaemic action of A-4166 showed an earlier onset and a shorter duration than that of sulphonylureas. 6. Simultaneous measurement of plasma insulin levels revealed that the hypoglycaemic effect of A-4166 was caused by a rapid-onset and brief burst of insulin secretion. 7. The pharmacokinetic profile of A-4166 was consistent with the changes of the blood glucose and plasma insulin levels. 8. Although the in vitro insulin-releasing effect of A-4166 was similar to that of sulphonylureas, its hypoglycaemic effect was more rapid and shorter-lasting, associated with rapid absorption and clearance. Thus, A-4166 may be useful in suppressing postprandial hyperglycaemia in patients with non-insulin-dependent diabetes mellitus.

Comparative dose-related time-action profiles of glibenclamide and a new non-sulphonylurea drug, AG-EE 623 ZW, during euglycaemic clamp in healthy subjects

Insulin and glucose responses to glibenclamide were studied in comparison to a novel non-sulphonylurea drug (AG) by means of the euglycaemic clamp technique. Nine fasting male subjects were connected to a Biostator and 1.75, 3.5 or 7.0 mg glibenclamide or 1.0, 2.0 or 4.0 mg AG were given and blood glucose concentrations were clamped at 10% below basal values. Glucose infusion rates were registered over 10 h after administration of the tablet. Maximal glucose infusion rates after glibenclamide were 40% higher compared to AG (1.75 vs 1.0 mg, 3.5 vs 2.0 mg, 7.0 vs 4.0 mg, respectively) and were reached after 3-3.5 h for all doses. After glibenclamide, area under the glucose infusion curves and maximal incremental serum insulin responses were higher by 25-40% and by 30% compared to AG when low, medium and high doses of each drug were tested. However, a linear dose relationship was obtained for both drugs when the glucose infusion rate was plotted against the area under the insulin curve. In fact, both drugs were equipotent on a molecular weight basis. The hypoglycaemic index of both drugs (integrated glucose infusion rate divided by integrated insulin release) expressed per mumol of drug revealed a dose-dependent and parallel inverse curvilinear relation to increasing doses. This methodological approach allowed us to quantify and compare the metabolic effects of oral hypoglycaemic agents under standardised experimental conditions.

Treatment of non-insulin-dependent diabetes mellitus and its complications. A state of the art review

Non-insulin-dependent diabetes mellitus (NIDDM) is a major health problem which occurs predominantly in the older population; 16.8% of persons over age 65 years have NIDDM. The total health costs of NIDDM are in excess of $US20 billion annually. The primary objective in the treatment of NIDDM is to achieve normoglycaemia, without aggravating coexisting abnormalities. Common abnormalities include obesity, hypertension, retinopathy, nephropathy and neuropathies. Diet, and consequent bodyweight reduction, is the cornerstone of therapy for NIDDM. Total calorie intake should be limited, while the percentage of calories from carbohydrates should be increased and that from fats and cholesterol should be decreased. Exercise may also help to reduce bodyweight. Sulphonylurea drugs stimulate insulin secretion from beta-cells, and may be a useful adjunct to nonpharmacological therapy. Failure to respond to sulphonylurea drugs may be primary (25 to 30% of initially treated patients) or secondary (5 to 10% per year). It is not clear which is the most effective pharmacological intervention in such cases. Options include switching to or combining therapy with insulin, a biguanide, or other insulin-sparing antihyperglycaemic agents, e.g. alpha-glucosidase inhibitors, thiazolidinediones, chloroquine or hydroxychloroquine, or fibric acid derivatives such as clofibrate. Other experimental agents include the fatty acid oxidation inhibitors and dichloroacetate. Specific agents, such as antihypertensives, lipid lowering agents and sorbitol inhibitors, may be needed to prevent the complications arising from the spectrum of clinical and metabolic abnormalities which arise from insulin resistance.

Problems and pitfalls of sulphonylurea therapy in older patients

The prevalence of non-insulin-dependent diabetes mellitus (type II) increases with age, so that approximately half of all known patients in English-speaking countries are over 65 years of age. There is no reason to believe that the criteria for blood glucose control should be any less stringent for elderly patients unless they have a limited life expectancy. Sulphonylurea drugs remain an effective means of achieving blood glucose control after failure of dietary therapy alone in older patients. However, changes in normal metabolism of drugs with age and the development of other pathologies in elderly patients make it important that these drugs are prescribed with care. Severe symptomatic hypoglycaemia is the most serious adverse effect of sulphonylurea drugs and this becomes progressively more likely with increasing age, depending primarily on the substantial reduction of renal function with normal aging. Other adverse effects are much less commonly of clinical importance. To minimise the risk of hypoglycaemia, it is important that patients receive closely supervised dietary management with education about their disease for at least 3 months before sulphonylurea drugs are prescribed. In elderly patients a short-acting agent with no active metabolites should be used. As patients become older, those receiving long-acting agents can be changed to short-acting agents before problems arise. If blood glucose control appears satisfactory on treatment, then symptoms of hypoglycaemia should be sought. If control is poor, then the criteria for introduction of insulin, with appropriate education, do not differ from those in younger patients.

Does glibenclamide influence the clearance of insulin and glucose uptake in patients with type 2 diabetes mellitus?

Sulphonylureas have been proposed to decrease the clearance of insulin based on the finding that they increase peripheral insulin concentrations more than C-peptide concentrations. However, direct evidence for such an effect has so far been lacking. The aim of this study was to investigate whether glibenclamide affects clearance of insulin in Type 2 diabetic patients. Nine patients with Type-2 diabetes participated in the study. Insulin clearance and glucose metabolism was assessed with a 240 min euglycaemic insulin clamp in combination with infusion of somatostatin (400 micrograms h-1) to completely suppress endogenous insulin secretion. Either saline or glibenclamide was infused throughout the clamp in random order. During both the glibenclamide and the saline protocol the C-peptide level declined to < 0.07 nmol l-1 within 150 min, indicating that insulin secretion was completely suppressed. However, peripheral clamp insulin concentrations remained similar during both saline and glibenclamide protocols (3374 +/- 258 vs. 3350 +/- 265 pmol l-1 x 240 min, p = NS). There was no significant difference in the metabolic clearance rate of insulin during the glibenclamide compared to the saline experiment neither during the first 120 min (796 +/- 36 vs. 757 +/- 34 ml m-2min-1) nor during the last 2 h of the clamp (780 +/- 43 vs. 724 +/- 35 ml m-2min-1). Total glucose metabolism during the first two (14 +/- 2 vs. 15 +/- 2 mumol kg-1 min-1) and the last 2 h of the clamp was similar both during saline and glibenclamide infusions (27 +/- 4 vs. 28 +/- 4 mumol kg-1min-1).(ABSTRACT TRUNCATED AT 250 WORDS)