Fluid secretion by the malpighian tubules of the tsetse fly Glossina morsitans: the effects of ouabain, ethacrynic acid and amiloride.
The effects of three inhibitors of sodium transport on the secretion of fluid by the Malpighian tubules of Glossina morsitans have been observed. The cardiac glycoside, ouabain, affects neither the rate of secretion nor the sodium concentration of the fluid secreted when isolated tubules are bathed by solutions containing a range of sodium and potassium concentrations. Secretion is inhibited, however, by ethacrynic acid and amiloride. The results confirm that fluid secretion by the Malpighian tubules of this insect is dependent on the active transport of sodium ions and show that Na+/k+ exchange pumps are not involved in this process. (+info)
Activation of stimulus-specific serine esterases (proteases) in the initiation of platelet secretion. I. Demonstration with organophosphorus inhibitors.
The effect of organophosphorus inhibitors of serine esterases (proteases) on secretion from washed rabbit platelets was examined. Five noncytotoxic stimuli were employed: collagen, thrombin, heterologous anti-platelet antibody (in the absence of complement), rabbit C3 bound to zymosan, and platelet activating factor derived from antigen-stimulated, IgE-sensitized rabbit basophils. Diisoprophyl phosphofluoridate, three series of p-nitrophenyl ethyl phosphonates, and a series of cyclohexyl phenylalkylphosphonofluridates were all found to be inhibitory to the platelet secretion. These are irreversible inhibitors of serine proteases but in this system were only inhibitory if added to the platelets concurrently with the stimuli. Pretreatment of either the platelets or the stimuli with the inhibitors followed by washing, was without effect on the subsequent reaction. This suggested the involvement of stimulus-activatable serine proteases in the secretory process. The concept was supported by finding that nonphosphorylating phosphonates or hydrolyzed phosphonates or phosphonofluoridates were without inhibitory action. The effect of a series of phosphonates or phosphonoflouridates in inhibiting each stimulus exhibited a unique activity-structure profile. The demonstration of such unique profiles with four series of inhibitors for each of the five stimuli was interpreted as demonstrating that a specific activatable serine protease was involved in the platelet secretory response to each stimulus. (+info)
Pancreatic beta-cell-to-beta-cell interactions are required for integrated responses to nutrient stimuli: enhanced Ca2+ and insulin secretory responses of MIN6 pseudoislets.
The effect of cell-to-cell contact on Ca2+ influx and secretory responses in the beta-cell line MIN6 was studied using MIN6 pseudoislets, which are three-dimensional islet-like cell aggregates that develop when MIN6 cells are cultured for 6-8 days on gelatin. The formation of pseudoislets is dependent on the Ca2+-dependent adhesion molecule E-cadherin (E-CAD), since the process can be inhibited by incubation in the absence of Ca2+ or in the presence of an anti-E-CAD antibody. Glucose and alpha-ketoisocaproic acid (KIC) evoked a Ca2+ influx in only a small fraction of the MIN6 monolayer cells, whereas >80% of cell groups within the pseudoislets responded to both nutrients. In contrast, changes in the intracellular free Ca2+ concentration ([Ca2+]i) were observed in all or most monolayer cells or pseudoislet cell groups in response to physical or pharmacological depolarizing stimuli. No significant increase in insulin release was observed from MIN6 monolayer cells in response to nutrient or nonnutrient insulin secretagogues. Conversely, pseudoislets were found to respond significantly to both nutrients and nonnutrients. These results suggest that close cell-to-cell contact improves the functional responsiveness of MIN6 cells and that pseudoislets may therefore serve as a useful research model in the study of beta-cell function. (+info)
Exposure of human islets to cytokines can result in disproportionately elevated proinsulin release.
Infiltration of immunocytes into pancreatic islets precedes loss of beta cells in type 1 diabetes. It is conceivable that local release of cytokines affects the function of beta cells before their apoptosis. This study examines whether the elevated proinsulin levels that have been described in prediabetes can result from exposure of beta cells to cytokines. Human beta-cell preparations were cultured for 48 or 72 hours with or without IL-1beta, TNF-alpha, or IFN-gamma, alone or in combination. None of these conditions were cytotoxic, nor did they reduce insulin biosynthetic activity. Single cytokines did not alter medium or cellular content in insulin or proinsulin. Cytokine combinations, in particular IL-1beta plus IFN-gamma, disproportionately elevated medium proinsulin levels. This effect expresses an altered functional state of the beta cells characterized by preserved proinsulin synthesis, a slower hormone conversion, and an increased ratio of cellular proinsulin over insulin content. The delay in proinsulin conversion can be attributed to lower expression of PC1 and PC2 convertases. It is concluded that disproportionately elevated proinsulin levels in pre-type 1 diabetic patients might result from exposure of their beta cells to cytokines released from infiltrating immunocytes. This hormonal alteration expresses an altered functional state of the beta cells that can occur independently of beta-cell death. (+info)
Secretory responses to sympathetic stimulation of the cat's salivary glands in a state of resting secretion.
The secretory effect of sympathetic stimulation on the cat's submaxillary gland was augmented greatly when studied against a background of slow secretion evoked by parasympathetic stimulation at a low frequency and imitating the slow resting secretion normally present in the waking state. The sympathetic secretory threshold was markedly lowered, and even at low frequencies sympathetic stimulation caused a large, well-maintained response. After an alpha-adrenoceptor blocking drug sympathetic stimulation alone lost its secretory effect, but during resting secretion part of the accelerating effect was found to remain; this effect was elicited via beta-adrenoceptors. A marked secretory effect of sympathetic stimulation was also obtained during resting secretion in the parotid gland, where the sympathetic secretory effect is normally very small. (+info)
Sexual differentiation of oestradiol-LH positive feedback in a marsupial.
The surge of LH that induces ovulation in mammals showing spontaneous ovulation is precipitated by the positive feedback of increasing oestrogens from the developing follicles in the ovary. In eutherians, exogenous oestrogens can mimic this effect by eliciting an LH surge in females, but not usually in males. The absence of a positive LH response to eutherian males is either due to an acute suppression by the secretory products of the testes during adulthood or the permanent disabling of the system by testosterone during early development. This phenomenon is examined in tammar wallabies, Macropus eugenii. The results show that the oestradiol-LH positive feedback response is sexually dimorphic in this marsupial. A surge in plasma LH occurred between 15 and 28 h after injection of 2.5 micrograms oestradiol benzoate kg-1 in 13 of 16 intact females and 4 of 4 ovariectomized females, but in none of 11 intact males. Five females each implanted with a 100 mg testosterone pellet 3 months earlier failed to produce an LH surge. Four males castrated in adulthood and three adult males castrated before puberty also failed to show an LH surge. However, three males castrated 24-26 days after birth showed an unambiguous LH surge when challenged with oestradiol benzoate during adulthood. Thus, in tammar wallabies, the ability to generate an LH surge to oestradiol is a sexually dimorphic response that is suppressed in the male by the organizational effects of the testes in early life and presumably supplemented by an inhibitory effect of circulating testosterone in adulthood. (+info)
Compensation by pulsatile GnRH infusions for incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and castrated male pigs.
The aim of this study was to investigate incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and male pigs. Gilts (250 days old; n = 36), which had been ovariectomized 30 (OVX 30) or 100 days (OVX 100) before the start of treatment, were challenged i.m. with oestradiol benzoate and were either given no further treatment, fed methallibure to inhibit endogenous GnRH release or fed methallibure and given i.v. pulses of 100 or 200 ng GnRH agonist at 1 h intervals during the LH surge (48-96 h after oestradiol benzoate). The same treatments were applied to long-term orchidectomized male pigs (ORC, n = 23). In addition, one ORC group was not injected with oestradiol benzoate but was fed methallibure and given pulses of 200 ng GnRH agonist. Oestradiol benzoate alone induced an LH surge in the OVX 30 group only (5/6 gilts), methallibure suppressed (P < 0.05) oestradiol benzoate-induced LH secretion, while pulses of 100 ng GnRH agonist in animals fed methallibure produced LH surges in four of six OVX 30 and four of six OVX 100 gilts. The induced LH surges were similar to those produced by oestradiol benzoate alone in OVX 30 gilts. Pulses of 200 ng GnRH agonist produced LH surges in OVX 30 (6/6) and OVX 100 (6/6) gilts and increased the magnitude of the induced LH surge in OVX 100 gilts (P < 0.05 compared with 100 ng GnRH agonist or OVX 30 control). Pulses of 200 ng GnRH agonist also induced LH surge release in ORC male pigs (5/6), but were unable to increase LH concentrations in a surge-like manner in ORC animals that had not been given oestradiol benzoate, indicating that oestradiol increases pituitary responsiveness to GnRH. These results support the hypothesis that oestradiol must inhibit secretion of LH before an LH surge can occur. It is concluded that incompetence for oestradiol-induced LH surges in long-term ovarian secretion-deprived gilts and in male pigs is due to the failure of oestradiol to promote a sufficient increase in the release of GnRH. (+info)
Stimulation-induced factors which affect augmentation and potentiation of trasmitter release at the neuromuscular junction.
1. End-plate potentials (e.p.p.s) were recorded from frog sartorius neuromuscular junctions under conditions of decreased transmitter release to study the effect of repetitive stimulation on augmentation and potentiation of transmitter release. 2. The magnitudes and time constants of decay of augmentation and potentiation were determined both following a primary conditioning train and following an identical secondary conditioning train applied from 30 to 170 sec after the primary conditioning train. 3. The magnitude of augmentation following the secondary conditioning trains was increased over that following the primary conditioning trains even though augmentation, with a time constant of decay of about 7 sec, had decayed to insignificant levels before the onset of the secondary trains. This increase in augmentation was not due to a change in its rate of decay during the secondary trains. 4. The increased magnitude of augmentation can be described as arising from an expression factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1-6 +/- 1-2 (S.D. of observation) (the magnitude of augmentation is increased 2-6 times) and decays approximately exponentially with a time constant of 90 +/- 50 (S.D. of observation) sec. The expression factor thus decays about ten times slower than augmentation. 5. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 2-8 +/- 1-0 (S.D. of observation) times increase in the magnitude of the expression factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 6. The expression factor, while increasing the magnitude of augmentation, had little or no effect on the magnitude of potentiation or on trasmitter release in the absence of augmentation. The expression factor decayed about twice as slowly as potentiation. 7. The time constants characterizing the decay of potentiation were greater following the secondary conditioning trains than following the primary conditioning trains. 8. The increased time constant for the decay of potentiation can be described as arising from a time constant factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1-2 +/- 0-7 (S.D. of observation) (the time constant of potentiation is increased 2-2 times) and decays approximately exponentially with a time constant of 130 +/- 45 (S.D. of observation) sec. The time constant factor decayed about three times slower than potentiation. 9. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 1-6 +/- 0-8 (S.D. of observation) times increase in the magnitude of the time constant factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 10... (+info)