Stimulation of Ca2+ influx in alpha T3-1 gonadotrophs via the cAMP/PKA signaling system

Longitudinal study on thyroid function in patients with thalassemia major: High incidence of central hypothyroidism by 18 years

INTRODUCTION

Primary hypothyroidism is one of the most frequent complications observed in-patients suffering from thalassemia. We investigated and reviewed the thyroid function in all thalassemic patients attending the Pediatric Endocrine Clinic of Hamad Medical Center, Doha, Qatar during the last 10 years of follow-up.

PATIENTS AND METHODS

A total of 48 patients with ί-thalassemia major between 5 years and 18 years of age. Thyroid dysfunction was defined as follows: Overt hypothyroidism (low Free thyroxine [FT4] and increased thyroid-stimulating hormone [TSH] levels >5 μIU/ml); subclinical hypothyroidism (normal FT4, TSH between 5 μIU/ml and 10 μIU/ml) and central (secondary) hypothyroidism (low FT4 and normal or decreased TSH).

RESULTS

A total of 48 patients (22 males and 26 females) completed a 12 year-period of follow-up. During this period, hypothyroidism was diagnosed in 17/48 (35%) of patients. There was no significant difference in the prevalence in males 7/22 (32%) versus females 10/26 (38%). Sixteen of the patients had hypothyroidism after the age of 10 years (94%). The prevalence of overt hypothyroidism had risen from 0% at the age of 7 years to 35% at the age of 18 years. None of the patients had high anti-thyroperoxidase antibody titers. Out of 17 patients, 13 patients with hypothyroidism had normal or low TSH level (not appropriately elevated) indicative of defective hypothalamic pituitary response to low FT4 (central hypothyroidism). Three patients (6.3%) had subclinical hypothyroidism (TSH between 5 uIU/ml and 10 uIU/ml and normal FT4). The general trend of FT4 level showed progressive decrease over the 12 years, whereas, TSH levels did not show a corresponding increase. These data suggested defective hypothalamic pituitary thyroid axis involving both TSH and FT4 sretion in patients with thalassemia major over time. There was a significant negative correlation between serum ferritin and FT4 (r = -0.39, P = 0.007), but no correlation was found between ferritin and TSH.

CONCLUSIONS

Worsening of thyroid function was observed in 35% of the studied thalassemic patients by the age of 18 years. The lack of proper increase of TSH in response to the low circulating levels of FT4 in 13/17 (76%) of these patients indicates a relatively high incidence of defective pituitary thyrotrophic function in these patients.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Role of L-type Ca2+ channels in iron transport and iron-overload cardiomyopathy

Excessive body iron or iron overload occurs under conditions such as primary (hereditary) hemochromatosis and secondary iron overload (hemosiderosis), which are reaching epidemic levels worldwide. Primary hemochromatosis is the most common genetic disorder with an allele frequency greater than 10% in individuals of European ancestry, while hemosiderosis is less common but associated with a much higher morbidity and mortality. Iron overload leads to iron deposition in many tissues especially the liver, brain, heart and endocrine tissues. Elevated cardiac iron leads to diastolic dysfunction, arrhythmias and dilated cardiomyopathy, and is the primary determinant of survival in patients with secondary iron overload as well as a leading cause of morbidity and mortality in primary hemochromatosis patients. In addition, iron-induced cardiac injury plays a role in acute iron toxicosis (iron poisoning), myocardial ischemia–reperfusion injury, Friedreich ataxia and neurodegenerative diseases. Patients with iron overload also routinely suffer from a range of endocrinopathies, including diabetes mellitus and anterior pituitary dysfunction. Despite clear connections between elevated iron and clinical disease, iron transport remains poorly understood. While low-capacity divalent metal and transferrin-bound transporters are critical under normal physiological conditions, L-type Ca²⁺ channels (LTCC) are high-capacity pathways of ferrous iron (Fe²⁺) uptake into cardiomyocytes especially under iron overload conditions. Fe²⁺ uptake through L-type Ca²⁺ channels may also be crucial in other excitable cells such as pancreatic beta cells, anterior pituitary cells and neurons. Consequently, LTCC blockers represent a potential new therapy to reduce the toxic effects of excess iron.

Effect of short-term cyclic stretch on sodium pump activity in aortic smooth muscle cells

We previously demonstrated that expression of both the alpha1- and alpha2-subunits of Na+-K+-ATPase is elevated after a 2- to 4-day cyclic stretch in aortic smooth muscle cells. In this study, we determined the effect of short-term (2-30 min) cyclic stretch on the activity of the Na pump and investigated possible mechanisms that may be involved in the action of stretch. Na pump activity was significantly increased above the baseline activity between 2 and 30 min of stretch. This effect of stretch was reversible within 1 h. Intracellular Na was also elevated at corresponding time points. Blocking the entry of Na with Gd and amiloride did not affect the stretch-induced increase in Na pump activity. Inhibition of protein kinase A (PKA) activity attenuated the effect of stretch on the Na pump. Furthermore, inhibition of polymerization of actin and phosphatidylinositol 3-kinase (PI3K) activity prevented the action of stretch on Na pump activity. We conclude that the stimulation of the Na pump in response to cyclic stretch requires the integrity of the actin cytoskeleton as well as the activity of PI3K, which has a role in intracellular vesicular trafficking. PKA may also be involved in this effect of stretch on Na pump.

Pituitary adenylate cyclase-activating polypeptide and melatonin in the suprachiasmatic nucleus: effects on the calcium signal transduction cascade

The suprachiasmatic nucleus (SCN) harbors an endogenous oscillator generating circadian rhythms that are synchronized to the external light/dark cycle by photic information transmitted via the retinohypothalamic tract (RHT). The RHT has recently been shown to contain pituitary adenylate cyclase-activating polypeptide (PACAP) as neurotransmitter/neuromodulator. PACAPergic effects on cAMP-mediated signaling events in the SCN are restricted to distinct time windows and sensitive to melatonin. In neurons isolated from the SCN of neonatal rats we investigated by means of the fura-2 technique whether PACAP and melatonin also influence the intracellular calcium concentration ([Ca2+]i). PACAP elicited increases of [Ca2+]i in 27% of the analyzed neurons, many of which were also responsive to the RHT neurotransmitters glutamate and/or substance P. PACAP-induced changes of [Ca2+]i were independent of cAMP, because they were not mimicked by forskolin or 8-bromo-cAMP. PACAP caused G-protein- and phospholipase C-mediated calcium release from inositol-trisphosphate-sensitive stores and subsequent protein kinase C-mediated calcium influx, demonstrated by treatment with GDP-beta-S, neomycin, U-73122, calcium-free saline, thapsigargin, bisindolylmaleimide, and chelerythrine. The calcium influx was insensitive to antagonists of voltage-gated calcium channels of the L-, N-, P-, Q- and T-type (diltiazem, nifedipine, verapamil, omega-conotoxin, omega-agatoxin, amiloride). Immunocytochemical characterization of the analyzed cells revealed that >50% of the PACAP-sensitive neurons were GABA-immunopositive. Our data demonstrate that in the SCN PACAP affects the [Ca2+]i, suggesting that different signaling pathways (calcium as well as cAMP) are involved in PACAPergic neurotransmission or neuromodulation. Melatonin did not interfere with calcium signaling, indicating that in SCN neurons the hormone primarily affects the cAMP signaling pathway.