DIABETES, VOL. 48, AUGUST 1999 1557 Deficiency of Phosphofructo-1-Kinase/Muscle Subtype in Humans Is Associated With Impairment of Insulin Secretory Oscillations Michael Ristow, Håkan Carlqvist, Judith Hebinck, Matthias Vorgerd, Wilhelm Krone, Andreas Pfeiffer, Dirk Müller-Wieland, and Claes-Göran Östenson In healthy humans, insulin is secreted in an oscillatory manner. While the underlying mechanisms generating these oscillations are not fully established, increasing evidence suggests a central role for phosphofructo-1- kinase/muscle subtype (PFK1-M), which also serves as the predominantly active PFK1 subtype in the pancreatic b-cell. The fact that normal oscillatory secretion is impaired in subjects with impaired glucose tolerance and healthy relatives of patients with type 2 diabetes suggests that this defect may be involved in the secre- tory dysfunction. To evaluate a possible link between inherited PFK1-M deficiency in humans (Tarui’s disease or glycogenosis type VII) and altered insulin oscilla- tions, in vivo studies were performed. We determined basal insulin oscillations during 2 h of frequent plasma sampling in two related teen-aged individuals with homozygous and heterozygous PFK1-M deficiency com- pared with nondeficient, unrelated control subjects. As predicted by the underlying hypothesis, normal oscilla- tions in insulin secretion were completely abolished in the individual with homozygous deficiency of PFK1-M and significantly impaired in the heterozygous individ- ual, as shown by spectral density and autocorrelation analyses. Thus, deficiency of PFK1-M subtype in humans appears to be associated with an impaired oscil- latory insulin secretion pattern and may contribute to the commonly observed secretion defects occurring in type 2 diabetes. Diabetes 48:1557–1561, 1999 W hile the observation of oscillatory insulin secre- tion was first described decades ago (1–4), its relevance to the pathogenesis of type 2 diabetes was repeatedly shown in later years, as impaired oscillations were proposed to be an early step in the development of type 2 diabetes (5,6). Thus, the origin of reduced pulsatility may be one important factor contributing to the impaired b-cell function observed in type 2 diabetes (5–8). Recent studies by Tornheim (8) and others have demonstrated that pulsatile insulin secretion may be linked to glycolytic oscillations in the pancreatic b-cell, defining the central stimulus-secretion coupling process. In addition to the b-cell–specific function of glucokinase, which has been shown to be associated with a maturity-onset diabetes of the young subtype (9), the rate-limiting step in glycolysis is the phosphorylating enzyme phosphofructo-1-kinase (PFK1) (EC 2.7.1.11) (10). PFK1 occurs in humans as three distinct subtypes encoded on different chromosomes. According to their predominant expression, these subtypes were named muscle (PFK1-M), liver, and platelet subtypes (11). PFK1-M has been shown to be regulated autocatalytically in an AMP- dependent oscillatory manner in muscle (12) as well as in pan- creatic b-cells (13). Furthermore, PFK1-M has been demon- strated to be the predominantly active subtype in both of these tissues (11,13). Thus, oscillations in glycolysis have been proposed to generate changes in the ATP/ADP ratio of the b-cell, leading to regulation of K ATP channels triggering oscillatory insulin secretion (8,13). While this coupling of PFK1-M–dependent ATP oscillations to pulsatile insulin secretion appears to be a convincing hypothesis supported by experimental evidence, direct proof—obtained by a PFK1-M knockout model, for instance—is lacking (8). We therefore decided to study humans with an inherited deficiency of PFK1-M, a rarely diagnosed (frequency 1:100,000 [14]) disorder called glycogenosis type VII or Tarui’s disease (15). The clinical features of this disease include excess glycogen storage, myopathy, hemolysis, and lack of exercise-induced lactate increase in peripheral venous blood (16,17). While only homozygously affected individuals exhibit these features, heterozygous deficiency is asympto- matic and detectable only by measuring reduced PFK1 activ- ity in red blood cells (RBCs). Recently, we demonstrated that homozygous PFK1-M deficiency predisposes to type 2 dia- betes (18). Individuals with homozygous PFK1-M deficiency exhibited a loss of intravenous glucose–induced first-phase insulin secretion, while both homo- and heterozygous PFK1-M deficiency led to peripheral insulin resistance. To gain a better understanding of a possible link between PFK1-M activity and the regulation of K AT P channels in the pancreatic b-cell, we evaluated alterations in basal oscillatory insulin secretion in a naturally occurring human knockout model for PFK1-M. In the present study, we found a significant impair- ment in the pulsatility of insulin secretion in these patients, sug- gesting a link between PFK1-M activity and insulin oscillations. From Klinik II und Poliklinik für Innere Medizin (M.R., W.K., D.M.-W.), Universität zu Köln, Cologne; Medizinische Klinik (J.H., A.P.) and Neurolo- gische Klinik, Muskelzentrum (M.V.), Bergmannsheil, Ruhr-Universität Bochum, Germany; and Karolinska Institutet (H.C., C.-G.Ö.), Department of Molecular Medicine, Endocrine and Diabetes Unit, Rolf Luft Center for Diabetes Research, Stockholm, Sweden. Address correspondence and reprint requests to M. Ristow, Harvard Medical School, Joslin Diabetes Center, Research Division, Section on Cellular and Molecular Physiology, Room 610, One Joslin Place, Boston, MA 02215. E-mail: michael.ristow@uni-koeln.de. Received for publication 6 November 1998 and accepted in revised form 20 April 1999. IVGTT, intravenous glucose tolerance test; OGTT, oral glucose tolerance test; PFK1, phosphofructo-1-kinase; PFK1-M, phosphofructo-1-kinase/ muscle subtype; RBC, red blood cell; RIA, radioimmunoassay.