Familial orthostatic tachycardia due to norepinephrine transporter deficiency.

UNLABELLED: Orthostatic intolerance (OI) or postural tachycardia syndrome (POTS) is a syndrome primarily affecting young females, and is characterized by lightheadedness, palpitations, fatigue, altered mentation, and syncope primarily occurring with upright posture and being relieved by lying down. There is typically tachycardia and raised plasma norepinephrine levels on upright posture, but little or no orthostatic hypotension. The pathophysiology of OI is believed to be very heterogeneous. Most studies of the syndrome have focused on abnormalities in norepinephrine release. Here the hypothesis that abnormal norepinephrine transporter (NET) function might contribute to the pathophysiology in some patients with OI was tested. In a proband with significant orthostatic symptoms and tachycardia, disproportionately elevated plasma norepinephrine with standing, impaired systemic, and local clearance of infused tritiated norepinephrine, impaired tyramine responsiveness, and a dissociation between stimulated plasma norepinephrine and DHPG elevation were found. Studies of NET gene structure in the proband revealed a coding mutation that converts a highly conserved transmembrane domain Ala residue to Pro. Analysis of the protein produced by the mutant cDNA in transfected cells demonstrated greater than 98% reduction in activity relative to normal. NE, DHPG/NE, and heart rate correlated with the mutant allele in this family. CONCLUSION: These results represent the first identification of a specific genetic defect in OI and the first disease linked to a coding alteration in a Na+/Cl(-)-dependent neurotransmitter transporter. Identification of this mechanism may facilitate our understanding of genetic causes of OI and lead to the development of more effective therapeutic modalities.

Alpha- and beta-cell responses to small changes in plasma glucose in the conscious dog.

The responses of the pancreatic alpha- and beta-cells to small changes in glucose were examined in overnight-fasted conscious dogs. Each study consisted of an equilibration (-140 to -40 min), a control (-40 to 0 min), and a test period (0 to 180 min), during which BAY R3401 (10 mg/kg), a glycogen phosphorylase inhibitor, was administered orally, either alone to create mild hypoglycemia or with peripheral glucose infusion to maintain euglycemia or create mild hyperglycemia. Drug administration in the hypoglycemic group decreased net hepatic glucose output (NHGO) from 8.9 +/- 1.7 (basal) to 6.0 +/- 1.7 and 5.8 +/- 1.0 pmol x kg(-1) x min(-1) by 30 and 90 min. As a result, the arterial plasma glucose level decreased from 5.8 +/- 0.2 (basal) to 5.2 +/- 0.3 and 4.4 +/- 0.3 mmol/l by 30 and 90 min, respectively (P < 0.01). Arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin decreased (P < 0.01) from 78 +/- 18 and 90 +/- 24 to 24 +/- 6 and 12 +/- 12 pmol/l over the first 30 min of the test period and decreased to 18 +/- 6 and 0 pmol/l by 90 min, respectively. The arterial glucagon levels and the hepatic portal-arterial difference in plasma glucagon increased from 43 +/- 5 and 4 +/- 2 to 51 +/- 5 and 10 +/- 5 ng/l by 30 min (P < 0.05) and to 79 +/- 16 and 31 +/- 15 ng/l by 90 min (P < 0.05), respectively. In euglycemic dogs, the arterial plasma glucose level remained at 5.9 +/- 0.1 mmol/l, and the NHGO decreased from 10 +/- 0.6 to -3.3 +/- 0.6 pmol x kg(-1) x min(-1) (180 min). The insulin and glucagon levels and the hepatic portal-arterial differences remained constant. In hyperglycemic dogs, the arterial plasma glucose level increased from 5.9 +/- 0.2 to 6.2 +/- 0.2 mmol/l by 30 min, and the NHGO decreased from 10 +/- 1.7 to 0 pmol x kg(-1) x min(-1) by 30 min. The arterial plasma insulin levels and the hepatic portal-arterial difference in plasma insulin increased from 60 +/- 18 and 78 +/- 24 to 126 +/- 30 and 192 +/- 42 pmol/l by 30 min, after which they averaged 138 +/- 24 and 282 +/- 30 pmol/l, respectively. The arterial plasma glucagon levels and the hepatic portal-arterial difference in plasma glucagon decreased slightly from 41 +/- 7 and 4 +/- 3 to 34 +/- 7 and 3 +/- 2 ng/l during the test period. These data show that the alpha- and beta-cells of the pancreas respond as a coupled unit to very small decreases in the plasma glucose level.

Modified structure of the human serotonin transporter promoter.

Recently, several studies have reported an association between anxiety traits, affective disorders and autism and alleles of a functional promoter polymorphism (5HTT-LPR) in the human serotonin transporter (5HTT, SERT).1-3 The mechanistic basis for allelic differences in transporter transcription are presently unknown. To explore this issue, we cloned the human 5HTT promoter region from a PAC genomic library and now describe an unreported 381-bp insert between the polymorphic region and the transcription start site. We verified the presence of this novel sequence by Southern hybridization of genomic digests and PCR amplifications from multiple unrelated individuals. Sequence analysis of the novel region reveals a number of canonical transcription factor binding sites (eg AP1, Elk1, NFkappaB) that may be important in controlling the response of the 5HTT gene to regulatory factors. PCR studies of genomic templates reveal a low level of amplification of a deleted template matching the size of the originally reported 5HTT promoter. This deleted template is absent from PAC amplifications, suggesting that the human 5HTT promoter may exhibit in vivo instability. Molecular Psychiatry (2000) 5, 110-115.

Orthostatic intolerance and tachycardia associated with norepinephrine-transporter deficiency.

BACKGROUND: Orthostatic intolerance is a syndrome characterized by lightheadedness, fatigue, altered mentation, and syncope and associated with postural tachycardia and plasma norepinephrine concentrations that are disproportionately high in relation to sympathetic outflow. We tested the hypothesis that impaired functioning of the norepinephrine transporter contributes to the pathophysiologic mechanism of orthostatic intolerance. METHODS: In a patient with orthostatic intolerance and her relatives, we measured postural blood pressure, heart rate, plasma catecholamines, and systemic norepinephrine spillover and clearance, and we sequenced the norepinephrine-transporter gene and evaluated its function. RESULTS: The patient had a high mean plasma norepinephrine concentration while standing, as compared with the mean (+/-SD) concentration in normal subjects (923 vs. 439+/-129 pg per milliliter [5.46 vs. 2.59+/-0.76 nmol per liter]), reduced systemic norepinephrine clearance (1.56 vs. 2.42+/-0.71 liters per minute), impairment in the increase in the plasma norepinephrine concentration after the administration of tyramine (12 vs. 56+/-63 pg per milliliter [0.07 vs. 0.33+/-0.37 pmol per liter]), and a disproportionate increase in the concentration of plasma norepinephrine relative to that of dihydroxyphenylglycol. Analysis of the norepinephrine-transporter gene revealed that the proband was heterozygous for a mutation in exon 9 (encoding a change from guanine to cytosine at position 237) that resulted in more than a 98 percent loss of function as compared with that of the wild-type gene. Impairment of synaptic norepinephrine clearance may result in a syndrome characterized by excessive sympathetic activation in response to physiologic stimuli. The mutant allele in the proband's family segregated with the postural heart rate and abnormal plasma catecholamine homeostasis. CONCLUSIONS: Genetic or acquired deficits in norepinephrine inactivation may underlie hyperadrenergic states that lead to orthostatic intolerance.

Orthostatic intolerance: emerging genetic and environmental etiologies.

Orthostatic intolerance (OI) is common after space flight and resembles the disabling idiopathic orthostatic intolerance commonly observed in otherwise healthy young individuals. OI can arise from reduced sympathetic nervous system activity and, paradoxically, also from increased sympathetic nervous system activity. Patients with early manifestations of pure autonomic failure demonstrate an etiology based upon reduced sympathetic nervous system activity. Patients with hyperadrenergia demonstrate an etiology based on increased sympathetic nervous activity. For many years, we have recognized that the microgravity environment induces adaptation in the cardiovascular system and its autonomic control mechanisms that lead to the presence of OI on return to gravity. Understanding the nature of OI in astronauts returning from space as well as in the relevant patient population on earth has been a priority of Vanderbilt's Center for Space Physiology and Medicine in recent years. A major purpose of the autonomic experiment in the Neurolab mission was to identify whether the OI experienced by astronauts on return to earth was best explained by a hypoadrenergic or hyperadrenergic state. To address this question, we analyzed sympathetic nervous system activity inflight by 1) measurement of plasma catecholamines; 2) assessment of peroneal microneurographic sympathetic nerve traffic; and 3) assessment of norepinephrine spillover and clearance during infusion of tritiated norepinephrine. These studies documented a slight (five bursts per minute) increase in muscle sympathetic nerve activity, a 200 pg/ml increase in plasma norepinephrine level, and a 350 ng/min increase in norepinephrine clearance. Plasma norepinephrine and norepinephrine spillover and clearance were also raised on recovery day. These data indicate that enhanced sympathetic activation, rather than reduced sympathetic activation, accompanies the orthostatic intolerance following microgravity.