Modulation of ryanodine receptor Ca2+ channels (Review).

Ryanodine-sensitive Ca2+ release channels (ryanodine receptors, RyRs) play a crucial role in the mobilization of Ca2+ from the sarcoplasmic reticulum (SR) during the excitation-contraction coupling of muscle cells. In skeletal muscle, depolarization of transverse tubules activates the RyR, whereas in cardiac muscle, a Ca2+ influx through an L-type Ca2+ channel activates the RyR. The RyR is also activated by caffeine, a low concentration (<10 µM) of ryanodine or cyclic ADP-ribose. RyR activity is inhibited by Mg2+, ruthenium red, or higher concentrations (≥100 µM) of ryanodine. The activity of RyR channels is modulated by phosphorylation and by associated proteins, including calmodulin (CaM), calsequestrin (CSQ) and FK506-binding proteins (FKBPs). In muscle cells, apoCaM (Ca2+-free CaM) activates the RyR channel, and Ca2+ CaM (Ca2+-bound CaM) inhibits the channel. CSQ can bind approximately 40 moles of Ca2+/mole of CSQ in the SR lumen of muscle cells, and interacts functionally with RyR protein. When the RyR is stimulated, Ca2+ released from the lumen is dissociated from the CSQ- Ca2+ complex. A 12-kDa or 12.6-kDa FK506-binding protein (FKBP12 or FKBP12.6, respectively) is associated with RyR protein. When FKBP12 or FKBP12.6 is dissociated from the FKBP-RyR complex, the RyR is modulated (activated). Phosphorylation of the RyR by cAMP-dependent protein kinase (PKA) and Ca2+/calmodulin-dependent protein kinase II modulates the channel. PKA phosphorylation of the RyR on the skeletal and cardiac muscle SR dissociates FKBP12 or FKBP12.6 from the RyR complex. This review deals with the modulation mechanisms of RyR proteins by associated proteins and phosphorylation.

Effects of FK506 on ca release channels (review).

Tacrolimus (FK506), which was isolated from the fermentation broth of Streptomyces tsukubaensis No. 9993, has an immunosuppressive effect. In T-lymphocytes, FK506 binds to the intracellular receptor, a 12-kDa FK506-binding protein (FKBP12). The FK506-FKBP12 complex binds to the phosphatase calcineurin (CN) and inhibits the activity of CN. By inhibition of the activity of CN, dephosphorylation of a nuclear factor of activated T-cells (NFAT) is inhibited, and translocation of the NFAT to the nucleus is suppressed. Thereby, the production of T-cell-derived mediators such as interleukin 2 (IL-2) is inhibited, and the proliferation of cytotoxic T-cells is suppressed. In muscle cells, FKBP12 and FKBP12.6 are associated with ryanodine-sensitive Ca(2+) release channels (ryanodine receptors: RyRs) on the skeletal and cardiac muscle sarcoplasmic reticulum (SR), respectively. FK506 modulates the RyR by dissociating FKBP12 or FKBP12.6 from the RyR complex. FKBP12 is also associated with inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) release channels (IP(3) receptors: IP(3)Rs) on the endoplasmic reticulum (ER) of non-muscle cells. The IP(3)R-FKBP12 complex binds to CN, which dephosphorylates the protein kinase C (PKC) phosphorylation site on the receptor. When FKBP12 is dissociated from the IP(3)R complex by FK506, CN is also dissociated from the IP(3)R. Thereby, the IP(3)R is phosphorylated by PKC, and the receptor is modulated. Recently, it was found that FK506 itself induces Ca(2+) release through RyRs in some tissues.

FK506 induces biphasic Ca2+ release from microsomal vesicles of rat pancreatic acinar cells.

The effect of the immunosuppressant drug FK506 on microsomal Ca2+ release was investigated in rat pancreatic acinar cells. When FK506 (0.1-200 microM) was added to the microsomal vesicles at a steady state of ATP-dependent 45Ca2+ uptake, FK506 caused a dose-dependent and a biphasic release of 45Ca2+. Almost 10% of total 45Ca2+ uptake was released at FK506 concentrations up to 10 microM (Km=0.47 microM), and 60% of total 45Ca2+ uptake was released at FK506 concentrations over 10 microM (Km=55 microM). Preincubation of the vesicles with cyclic ADP-ribose (cADPR, 0.5 microM) increased the FK506 (< or =10 microM)-induced 45Ca2+ release (Ozawa T, Biochim Biophys Acta 1693: 159-166, 2004). Preincubation with heparin (200 microg/ml) resulted in significant inhibition of the FK506 (30 microM)-induced 45Ca2+ release. Subsequent addition of inositol 1,4,5-trisphosphate (IP3, 5 microM) after FK506 (100 microM)-induced 45Ca2+ release did not cause any release of 45Ca2+. These results indicate that two types of FK506-induced Ca2+ release mechanism operate in the endoplasmic reticulum of rat pancreatic acinar cells: a high-affinity mechanism of Ca2+ release, which involves activation of the ryanodine receptor, and a low-affinity mechanism of Ca2+ release, which involves activation of the IP3 receptor.

Elucidation of the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells: modulation by cyclic ADP-ribose and FK506.

The effects of cyclic ADP-ribose (cADPR) and the immunosuppressant drug FK506 on microsomal Ca2+ release through a ryanodine-sensitive mechanism were investigated in rat pancreatic acinar cells. After a steady state of 45Ca2+ uptake into the microsomal vesicles, ryanodine or caffeine was added. Preincubation of the vesicles with cADPR (0.5 microM) shifted the dose-response curve of ryanodine- or caffeine-induced 45Ca2+ release from the vesicles to the left. Preincubation with cADPR shifted the dose-response curve of the FK506-induced 45Ca2+ release upward. Preincubation with FK506 (3 microM) shifted the dose-response curve of the ryanodine- or caffeine-induced 45Ca2+ release to the left by the same extent as that in the case of cADPR. FK506 shifted the dose-response curve of the cADPR-induced 45Ca2+ release upward. The presence of both cADPR and FK506 enhanced the ryanodine (30 microM)- or caffeine (10 mM)-induced 45Ca2+ release by the same extent as that in the case of cADPR alone or FK506 alone. These results indicate that cADPR and FK506 modulate the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells by increasing the ryanodine or caffeine sensitivity to the mechanism. In addition, there is a possibility that the mechanisms of modulation by cADPR and FK506 are the same.

[A 50-year history of new drugs in Japan-the development and progress of anti-diabetic drugs and the epidemiological aspects of diabetes mellitus].

The development and progress of antidiabetic drugs (e.g., insulin preparations and hypoglycemic drugs) are retrospectively investigated in Japan. Their influences on the treatment of diabetes mellitus (DM) and its epidemiological aspects are also discussed. 1) Insulin preparations: Insulin was introduced for DM therapy in 1925, two or three years after its discovery in Canada. The preparations were raw extracts of bovine or porcine pancreas. These did not prevail widely in Japan because of the low incidence of DM before World Wan II. After the war, a shortage of mammalian materials compelled the use of fish pancreatic tissues such as bonito and/or tuna for insulin production. Insulin infection, so-called regular insulin, was first promoted in the 6th "Pharmacopoeia Japonica" (JP6) in 1951 and has been maintained to the present edition (JP14, 2001). Although depot-type insulin preparations were developed in the USA and Europe during the war, the introduction of those preparations to Japan was delayed until 1951, when Protamine zinc insulin appeared. Globin zinc insulin and Isophane insulin were introduced for clinical use in 1952 and 1955, respectively. These were also adopted for JP7 (1961). Biphasic-type insulin, which has a rapid onset and long duration of activity, appeared in 1965. Purified preparations from bovine or porcine sources have been available since 1980, which might be a strong reason for the decrease in insulin allergy. Insulin from animal origin has been supplied for almost 60 years since its discovery. Amino acid sequences of insulins from various species of animals were determined by the pioneering studies of Sanger and his associates. Human insulin, which differs from porcine insulin by only one amino acid, was produced by Novo researchers in 1982 using a semi-synthetic method. Then the Lilly group soon succeeded in obtaining human insulin by recombinant DNA technology in the same year. Both products were introduced to Japan in 1985, and the recombinant products prevailed throughout the 1990s. Human insulin analogues (i.e., Insulin lispro and Insulin aspart) appeared in 2001. These are applied for after-meal glycosmia owing to their ultrarapid onset of activity. Self-injection by DM patients was legalized in 1981. To make the infection technique sure and easy, cartridge (pen-type) and disposable kit-type needles were devised in the 1990s. 2) Oral hypoglycemic drugs: Instead of the exclusive parenteral usage of insulins, there was also demand for oral dosage forms. The first of the sulfonyrlurea (SU) group, BZ-55, was used for DM clinically in 1955 in Germany. But it was soon withdrawn because of its antibacterial action. This led to the development of various SU groups. Tolbutamide (1956), chlorpropamide (1959), acetohexamide (1964) and tolazamide (1961) were introduced to Japan as first-generation SUs. Then glyclopyramide (Kyorin, 1965), glybenclamide (1971), gliclazide (1984) and glimepiride (1999) appeared as the second-generation SUs. These were used orally for Type 2 diabetes. Biguanide (BG) group, phenformin HC1 (1959), metformin HC1 (1961) and buformin HC1 (1961) had also been in use by oral treatment of Type 2 diabetes. SU appears to act by increasing the sensitivity of b-cells, which secrete insulin. BG probably exerts by increasing glucose transport across the membranes of target organs. 3) New types of antidiabetic drugs: a-Glucosidase inhibitors (i.e., acarbose: Bayer, 1993; and voglibose: Takeda, 1994) act on hyperglycemia after meals by decreasing glucose absorption. Thiazolidinedione compounds, such as troglitazone (Sankyo, 1995) and pioglitazone HC1 (Takeda, 1994) act by increasing the insulin sensitivity of the target tissues. These are useful for Type 2 DM patients when SUs are ineffective. Nevertheless, troglitazone was discontinued in 2000 due to severe liver damage. Nateglinide (Ajinomoto Co., 1999), which is a D-phenylalanine derivative acting similar to SUs, is useful orally for after-meal hyperglycemia of Type 2 diabetes. Epalrestat (Ono Yakuhin Co., 1992) is effective for diabetic neuropathy by reducing the formation of sorbitol. These anti-DM drugs were recently studied and developed in Japan. 4) The Japan Diabetes Society proposed a guideline on diagnostic criteria and treatment of diabetes mellitus (DM) in 1999 and revised it in 2002. DM is classified as insulin-dependent DM (Type l) and non-insulin dependent DM (Type 2). Type 1, juvenile onset DM, requires insulin therapy to prevent ketosis and to sustain life. Treatment of type 2, adult onset DM, is recommended as a step-by-step method, starting with dietary-exercise therapy, followed by oral hypoglycemic drugs and then insulin therapy. DM patients with complications should have a therapy devised to match their circumstances. 5) Epidemiological aspects: The mortality rate of DM compared to the time of drug appearance was traced from 1920 to 2000. The curve goes down slowly in the time frame of World War II, but rises from 1950 to 1970. The elevation could not be suppressed by the appearance of SUs, BGs or improved insulin preparations. The curve runs flat from 1980 to 1990, which might be related to the use of purified insulin or human insulin therapy. The mortality rate of DM indicates that death by hyperglycemic coma and other deaths resulting from complications are excluded. The survey of the principal cause of death by DM during the period of 1981-1990 indicates that the death rate due to hyperglycemic coma is only 1.7% of the total deaths caused by DM. The effect of drug therapy on all of the death resulting from DM is not detected. Hospital visitation and admission rates of the DM patients have been recorded since 1952 in Japan. This curve is rising continuously, and none of the antidiabetic drugs has been able to suppress it. These data show that the antidiabetic drugs relieve DM symptoms through their effective hypoglycemic actions, but that they cannot suppress the mortality rate of DM. It is possible that none of the drugs currently available can suppress the increasing tendency of DM patients.

[A 50-year year history of new drugs in Japan: the developments of antituberculosis drugs and their influences on the epidemiological aspects].

Drugs used in the treatment of tuberculosis (Tb) in Japan are investigated. Especially the chemotherapy for Tb and its influences on epidemiological aspects are discussed. 1. Various drugs were used for Tb patients before the World War II, but none was effective in curing this infectious disease. Creosote and guajacol groups were used frequently to relieve symptoms of pulmonary tuberculosis, but the disease could not cure itself. Because of the sacrifice of young patients, the mortality rate of Tb from 1935 to 1950 was ranked as the worst in Japan. So until the advent of chemotherapeutic drugs, Tb was known as the most formidable fatal plague. 2. Streptomycin (SM), the first effective chemotherapeutic drug, was imported into Japan and widely used from 1947. PAS in 1950 and isoniazid (INH) in 1952 were introduced to Tb therapy. The triple combination therapy of these drugs was considered the most favorable regimen for Tb from the 1950s to the early 1970s. Excellent results were obtained in this period. The mortality rate of Tb had dropped rapidly from its peak to half in 1952 and to a fourth in 1956 (Figs. 2, 3). 3. Several anti-Tb drugs, such as pyrazinamide ethionacide, ethambutol, and some antibiotics (kanamycin, cyloserine, and capreomycin) had been discovered and used in practice. These were not used singly, because of their weak clinical efficacies and severe side effects. They were mostly used to prevent the development of bacillus resistance to SM or INH. In the guidelines of the Japanese Society for Tuberculosis, in 1974. drugs used for Tb could be divided into two major categories: first-line and second-line groups. The second-line drugs included those that prevent a high resistance to the main (first-line) drugs. 4. Rifampicin (RFP), the most valuable drug for Tb, was introduced in therapy in Japan in 1971. RFP has a low incidence of severe side-effects, but because of the rapidity with which resistance may develop, it cannot be used alone. RFP in combination with INH is the most effective therapy for all forms of the disease. The guidelines, newly proposed in 1986 by the Japanese Society for Tuberculosis recommended the short 6-month course of treatment that used combination of RFP and INH. The advent of RFP had contributed to the cure of the individual patient, but it did not effect the mortality or the morbidity rate of Tb. Chemotherapy is the most effective means of suppressing tuberculosis, which was formerly nearly always fatal, but it could not completely eradicate the disease. Preventing the development of resistance to chemotherapeutic drugs might be a special problem.