Circadian changes in microtubules,synaptic ribbons and synaptic ribbon fields in the pinealocytes of the baboon (Papio ursinus)

Summary In baboons kept under controlled lighting conditions, microtubules (MT) are readily seen in the perikaryal cytoplasm and in the perivascular processes of pinealocytes. A significant increase in the number of MT, single synaptic ribbons (SR) and the formation of synaptic ribbon fields (RF, i.e. organelles which consist of multiple dense rodlets or plates, and vesicles), occur during the dark phase of a circadian light-dark cycle. MT may act as tracks for the oriented flow of vesicles derived from the smooth endoplasmic reticulum, to cytoplasmic sites where RF are being formed. The origin of the dense rodlets of RF remains unknown. Structural differences between SR and RF indicate that the latter organelles are not directly involved in impulse propagation between adjacent baboon pinealocytes. RF may function as storage organelles for some of the pineal secretory products which are formed in large amounts during the dark phase.     

The pineal gland of the gerbil,Meriones unguiculatus

Summary By means of morphometric analytical procedures, a diurnal rhythm in the cellular volume of gerbil pinealocytes was determined. This rhythm has been attributed primarily to a change in the cytoplasmic volume of the pinealocytes which is low during the daylight hours and increases to reach a peak during the middle of the dark period. At the ultrastructural level, six cytoplasmic components of the pinealocytes were found to exhibit a rhythm: free cytoplasm, smooth endoplasmic reticulum (SER), rough endoplasmic reticulum (RER) and ribosomes, secretory vesicles, microtubules, and mitochondria. The presumptive secretory vesicles and the microtubules reached a peak in volume one hour before lights-off. It is suggested that lights-on and lights-off both signal a decrease in size and/or number of the secretory vesicles. The SER and RER/ribosomes reached their peak volume one hour after lights-off which is interpreted as indicating a peak in indoleamine synthesis and protein synthesis, respectively. The volume of free cytoplasm exhibits two peaks; one occurs one hour before lights-off while the second peak occurs in the middle of the dark phase. It is suggested that, although part of the secretory product of the pinealocyte may be present in dense-cored vesicles, other locations could include the free cytoplasm and clear secretory vesicles.Supported by NSF grant #PCM 77-05734     

Synaptic ribbon populations in the pineal gland of the rhesus monkey (Macaca mulatta)

Summary Pinealocytes of rhesus monkeys that had been ovariectomized and given intramuscular injections of 250 g estradiol-benzoate for 3 consecutive days tended to have more synaptic ribbons (SR) and exhibited a significantly greater size of ribbon fields (RF) compared to untreated animals. These data are consistent with hypotheses that pinealocyte function in primates is altered by hormonal imbalances and that the SR participates in this response. RF were positioned in various parts of the cytoplasm and along the plasma membrane. Participation of SR in direct cell-to-cell contacts was suggested by the formation of densities along the plasma membrane. It is postulated that large RF serve as storage organelles and that the formation of RF results from division of pre-existing SR in each field. Reconstructions made from serial thin sections revealed that profiles of RF comprised separate SR that were not folded and sectioned along various planes.     

A special pattern of the smooth endoplasmic reticulum in the kidney of the snail Cryptomphalus aspersa.

A special structural pattern of the smooth endoplasmic reticulum (SER) has been observed in the kidney of the snail Cryptomphalus aspersa. Two types of cells (clear and dark) cover the foldings of the renal sac; the dark cells are by far the most numerous. A cisterna of SER enveloping the nucleus appears invariably in both types of cells, with no disruptions, or small ones (from 50 to 90 nm) along its profile. The layer of cytoplasm lodged between the external nuclear membrane and this cisterna is found invariably to be from 0-20 to 0-25 mum in width. Glycogen is abundant in the cytoplasm as alpha particles, and also in the nucleus, but as beta particles. It is noteworthy that absolutely no glycogen is present in the layer of cytoplasm lodged between the nuclear membrane and the surrounding SER envelope. Long profiles of SER are also observed closely approaching and parallel to the plasma membrane of the dark cells. Considering the role of SER in glycogen metabolism in the kidney of the snail, the possible function of these cisternae as a support system ofr enzymes involved in the metabolism of glucides is discussed.     

Prenatal development of "synaptic" ribbons in the guinea pig pineal gland

Pineal "synaptic" ribbons are a heterogeneous population of organelles. "Synaptic" ribbons (SR) sensu stricto, "synaptic" spherules (SS), and intermediate forms (IMF) are present. Their function and origin are unknown, and a knowledge of their prenatal development is lacking. Thus the pineal glands of prenatal, neonatal, and adult guinea pigs were prepared for electron microscopy. "Synaptic" ribbons were studied morphologically and quantitatively. The three categories of "synaptic" ribbons reported in adult pineal glands were also present in prenatal pineal glands. Their structural features, distribution, grouping, and composition patterns are similar to those in adults. "Synaptic" ribbons were first detected in pinealocytes of the distal region of a 42-day postcoitus (PC) pineal gland and were comparable with those in adults. They increased in number with age and reached a peak at 63 days PC, followed by a steep decline at 66 and 67 days PC. By day 69 PC, the numbers increased again and showed a dramatic increase after birth. Several true ribbon synapses were seen at day 63 PC between pinealocyte cell processes or between pinealocyte cell process and pinealocyte cell body. Since true ribbon synapses have not been found in adult guinea pig pinealocytes, their synaptic nature could have been lost during development. No precursors for the "synaptic" ribbons were found. The endoplasmic reticulum cisternae may be the origin for the ribbon vesicles because of their close association with the "synaptic" ribbons.     

Sporoderm development in Liriodendron chinense (Magnoliaceae): a probable role of the endoplasmic reticulum

The developmental events in the sporoderm and the cytoplasm of Liriodendron chinense microspore from the early tetrad stage until late free microspore stage were observed. Various forms of the endoplasmic reticulum (ER) and surprising unusual aggregates of ER seen during microspore development attract special attention. Being scanty at early and middle tetrad stage, while primexine matrix (glycocalyx) acquires well-defined form, the ER becomes distinct in unusual forms at the late tetrad stage. Thin long tubules with an osmiophilic contents, which cannot be compared with the tubular smooth endoplasmic reticulum (SER), undulate through the cytoplasm. Towards the end of the tetrad period when callose begins to disintegrate, and a distinct tectate-columellate pattern of the ectexine becomes evident, two new forms of the SER occur in the cytoplasm. Instead of single tubules observed previously, 3–tubuled aggregates meander through the cytoplasm, the middle tubule contains an osmiophilic substance. The second form of the SER looks like an ordinary tubular SER, but has ampoule-like dilations with dark granular contents. Later on the tubules undergo major changes: multi-tubuled aggregates of parallel tubules overcrowd the cytoplasm, the outer tubules of each aggregate carrying ribosomes. These aggregates undulate through the cytoplasm, branch, and are associated with lipid globules. The tips of many aggregates are pressed to the plasmalemma. The ontogenetic period of time of the presence of these ER aggregates, their structure and localization in the microspore cytoplasm allow me to assume that these ER aggregates synthesize sporopollenin precursors.     

Spatial relationship between the mental foramen and mandibular developing teeth in modern humans,chimpanzees, and hamadryas baboons

The mental foramen (MF) of adult human mandibles is characterized by its high position and posterosuperior opening orientation, compared with that of nonhuman primates. In this study, to examine when and how such interspecies variations in MF position/orientation are manifested in the context of dental development, positional relationships between the MF and nearby forming teeth (dc, dm1, C, P3) were assessed using CT‐scanned growth‐series mandibles of the following three species with various MF positions and anterior dental sizes: modern humans, chimpanzees, and hamadryas baboons. Results showed that modern humans have more antero‐superiorly located MF and dc than the two nonhuman samples during growth, whereas the MF and dm1 of hamadryas baboons are the most inferiorly positioned. Considering that the mandibular canal generally reaches the dc/dm1 position during infancy, the species differences in MF position can be attributed largely to the positions where the anterior deciduous teeth first appear. Specifically, the distinctive dental position of modern humans should stem eventually from the comparatively small anterior dental size. Furthermore, human MF position shifted slightly upwards with alveolar development, unlike in the nonhuman samples, accompanied by strong curvature and vertical elongation of the anterior mandibular canal. Meanwhile, the human MF shifted from a forward to a lateral/backward orientation in association with human‐specific growth‐related alveolar recession. The findings of this study, thus, collectively indicate that taxonomic variations in adult MF position/orientation can be interpreted partly by the positions of the surrounding developing teeth. Am J Phys Anthropol 154:594–603, 2014. © 2014 Wiley Periodicals, Inc.     

Olfactory mucosa ultrastructure in the short-tailed shrews,Blarina brevicauda and Blarina carolinensis

The olfactory mucosae of the northern short-tailed shrew, Blarina brevicauda, and the southern short-tailed shrew, Blarina carolinensis, were examined by light and electron microscopy. A well-developed olfactory epithelium was observed that included all of the cells necessary to provide for a sensitive olfactory system, suggesting that olfaction plays a major role in the behavior of these animals. There were no significant differences between the olfactory mucosae of these two species. The general features of the olfactory epithelium in these shrews were similar to those reported for several other macrosmatic mammals. A new type of supporting cell, called the light supporting cell, was observed in these shrews. The light supporting cell cytoplasm exhibited very little staining by light microscopy and had low electron density by transmission electron microscopy compared to that of the more common dark supporting cell. The light supporting cell had a convex apical surface with microvilli and lacked the large amounts of smooth endoplasmic reticulum (SER) typical of the apical cytoplasm of the dark supporting cell. In the lamina propria of the mucosa, the Bowman's glands consisted of two cell types, one with electron-lucent, alcian blue-positive granules, and the other with electron-dense PAS-positive granules. The cell with electron-lucent granules contained large amounts of SER and small clumps of rough ER. The cells with electron-dense granules had large amounts of RER and little SER.     

An ultrastructural study of the effect of a diabetogenic dose of alloxan on the secretory ameloblasts of the rat incisor

Sertoli cells of the ground squirrel (Spermophilus lateralis), a seasonal breeder, were examined by light and electron microscopy and their structure, particularly the organization of the cytoskeleton, was related to events that occur in the seminiferous epithelium during spermatogenesis. Among the events considered and described are the apical movement of elongate spermatids, withdrawal of residual cytoplasm from germ cells, transport of smooth endoplasmic reticulum (SER) between the base and apex of the Sertoli cells, and sperm release. These events are dramatically evident in this species because the seminiferous epithelium is thin, i.e., there are few germ cells, and both the germ cells and Sertoli cells are large. Sertoli cells of the ground squirrel have a remarkably well developed cytoskeleton. Microfilaments occur throughout the cell but are most evident in ectoplasmic specializations associated with junctions. Intermediate filaments occur around the nucleus, as a layer at the base of the cell, and adjacent to desmosome-like junctions with germ cells. Intermediate filaments, together with microtubules, are also abundant in regions of the cell involved with the transport of SER, in cytoplasm associated with elongate spermatids, and in processes that extend into the residual cytoplasm of germ cells. Our observations of ultrastructure are consistent with the hypothesis that Sertoli cell microtubules are involved with the movement of germ cells within the seminiferous epithelium, and further implicate these structures as possibly playing a role in the retraction of residual cytoplasm from germ cells and the intracellular transport of SER. The abundance and organization of intermediate filaments suggest that these cytoskeletal elements may also be involved with events that occur during spermatogenesis.     

Effect of photoperiod on pineal gland volume and pinealocyte size in the Chinese hamster, Cricetulus griseus

Male adult (200-day-old) Chinese hamsters (Cricetulus griseus) raised from weaning under either LD 16:8 or LD 8:16 were used. The pineal gland of the Chinese hamster consists of superficial (major) and deep (minor) components and a continuous, or interrupted, narrow parenchymal stalk interposed between them. The volume of the superficial pineal including the parenchymal stalk is greater under LD 16:8 than under LD 8:16. Under both photoperiods, pinealocytes in the superficial pineal have larger nuclei and more abundant cytoplasm than those in the deep pineal. Nuclei in the superficial pineal appear pale and usually have irregular profiles, whereas those in the deep pineal appear dark and have round profiles. In the superficial pineal, pinealocyte nuclei are larger, paler, and more irregular; and, in addition, nuclear density is lower under LD 16:8 than under LD 8:16. Similar, but less prominent, photoperiod-induced changes occur in the volume of the deep pineal, the size of pinealocytes, and pinealocyte nuclear morphology in the deep pineal. The results indicate that the development and differentiation of pinealocytes in both pineal portions may be advanced under long photoperiods and delayed under short photoperiods, although pinealocytes in the deep pineal may remain not fully differentiated even in adults. Since testicular weights and body weights are similar under both photoperiods, the photoperiod may exert marked influences on the development of the pineal gland without affecting reproductive activity and growth rates of animals.     

Effects of reserpine and p-chlorophenylalanine on the circadian rhythm of granulated vesicles in the pinealocytes of mice

Summary The number of granulated vesicles in mouse pinealocytes exhibit a distinct circadian rhythm which is abolished by superior cervical ganglionectomy. Since melatonin treatment markedly affects the number of pinealocytic granulated vesicles, it was suggested that a relationship may exist between norepinephrine, melatonin, and the synthesis and/or secretion of pinealocyte granulated vesicles. The present study was undertaken in an attempt to clarify this relationship. Mice were housed in an environmental chamber under controlled lighting (12L/12D), and were treated with either reserpine, a drug which depletes serotonin and norepinephrine, or p-chlorophenylalanine (p-CPA), an inhibitor of serotonin synthesis. They were sacrificed at various times over a twenty-four hour period, and granulated vesicles present in the pinealocytes were quantitated in thin cross sections through pericapillary areas. Reserpine treatment resulted in a marked increase in the number of granulated vesicles during the dark, but led to a slight decrease during the photoperiod. Treatment with p-CPA produced no significant effect. The results of this study do not support our previous theory that melatonin plays an important role in the regulation of the pinealocyte secretory process, but instead points more directly to the significant role that norepinephrine has in controlling pinealocyte secretion.Supported in part by N.I.H. Grant # HD 08759     

Actin-endoplasmic reticulum complexes inDrosera

In epidermal cells ofDrosera tentacles that have been preserved for ultrastructural analysis through high pressure freeze fixation and freeze substitution we describe the frequent occurrence of microfilament (MF)-endoplasmic reticulum (ER) complexes. These are found throughout the cytoplasm where they are observed in close association with the plasmalemma (PL), the tonoplast, nuclei, mitochondria, chloroplasts, and microbodies. The MF component of the complexes is identified as actin based on immunogold labelling with actin antibodies. The actin-ER complexes are prominent in the cortical cytoplasm. In this region a network of predominantly tubular ER occupies an intermediary position in which it associates closely with both the PL and the actin MFs. We suggest that the ER, especially those elements adjacent to the PL in the cortical cytoplasm, stabilizes the actin MFs and provides the necessary anchor against which the forces for cytoplasmic streaming are generated.Abbreviations CF chemical fixation - ER endoplasmic reticulum - FS freeze substitution - HPF high pressure freezing - MF microfilaments - MT microtubules - PL plasmalemma     

The pineal gland of the gerbil,Meriones unguiculatus

Morphometric analytical procedures were employed to study the pineal gland of the Mongolian gerbil following superior cervical ganglionectomy (SCGX). The purpose of this study was to define the effects of sympathetic denervation on the morphology of the gland at two time periods, 0500 and 1900 h (one hour before lights-on and lights-off, respectively). Fluorescence histochemistry was employed to determine catecholamine and indoleamine content in intact and denervated pineal glands. After SCGX, the pinealocytes decrease in size, concretions are prevented from forming, and the yellow fluorescence in the gland is lost. Following denervation a depression in the volume of most of the pinealocyte organelles, i.e., SER, RER/ribosomes, free cytoplasm, mitochondria and presumptive secretory vesicles, was also observed. However, synaptic ribbons increased in volume in the gerbils that had been killed at 1900 h. It appears that the sympathetic innervation to the pineal gland is a requirement for the presumptive secretory activity of the pinealocytes.     

Cytological differentiation of the interrenal tissue of the Japanese quail,Coturnix coturnix

Summary As reported for several other avian species there are clearly distinguishable subcapsular (SCZ) and inner (IZ) zones of interrenal tissue in the Japanese quail. The SCZ contains large columnar cells (type I) with rounded nuclei, polymorphic mitochondria with shelf-like cristae, and relatively small numbers of lipid droplets. The IZ contains two and possibly three types of cells. Type II consists of large columnar cells with moderately dense cytoplasm containing large numbers of lipid droplets and many rounded mitochondria with tubular cristae. Smooth endoplasmic reticulum (SER) and Golgi apparatus are well developed; coated vesicles occur in the Golgi area and at the cell surface. Type-III cells occur in IZ and especially in its more peripheral areas. They are columnar cells with strikingly clear cytoplasm (in comparison with type II) containing mitochondria with plate-like cristae and tubular SER. Type-IV cells are sparsely distributed in IZ and occur rarely in SCZ. Type IV may be a degenerating phase of type III.After adenohypophysectomy or section of portal vessels type-I cells atrophy somewhat with a decrease in lipid droplets; type-II cells, also atrophy with conspicuous increase in size and number of lipid droplets, enlargement of mitochondria, and gradual disappearance of SER; type-III cells decrease in number whereas type-IV cells increase.After injection of ACTH, type-I cells enlarge and their mitochondria, SER and Golgi apparatus become more conspicuous; there is a decrease in lipid droplets in type-II cells and a development of SER, polysomes and Golgi apparatus; there is also a decrease in lipid droplets and a development of SER in type-III cells after injection of 2IU ACTH and an almost complete disappearance of lipid droplets after 4IU ACTH; type-IV cells increase in number.The investigation reported herein was supported by Scientific Research Grants from the Ministry of Education of Japan to Professor Mikami; and by grants from the Japan Society for the Promotion of Science, the National Science Foundation (USA), and the Graduate School Fund of the University of Washington to Professor Farner     

Rearrangement of the close contact between the mitochondria and the sarcoplasmic reticulum in airway smooth muscle

The mitochondria and the sarcoplasmic reticulum (SR) are two major intracellular calcium-storing organelles that exhibit close functional interaction with each other. Close spatial association is believed to be important for their functional interaction. In this study, we have characterized the spatial relationship between the SR and the mitochondria in porcine tracheal smooth muscle cells (TSMC) under different conditions. By examining the cross-section of unstimulated TSMC with electron microscopy, we found that 99.4 +/- 0.5% of the mitochondria seen on random cross-sections were situated within 30 nm of the SR and that 82.2 +/- 6.7% of the mitochondria were completely enveloped by the SR network. Overall, 48.0 +/- 3.5% of the mitochondrial outer membrane was within 30 nm with the SR. After stimulation of the TSMC with acetylcholine (ACh) or 80 mM [K(+)] solution 97.0 +/- 2.1% and 98.6 +/- 1.4% of the mitochondria observed were situated within 30 nm of the SR, respectively. However, the proportion of the mitochondria that was completely enveloped by the SR was significantly reduced to 12.2 +/- 5.9% in ACh-stimulated cells and 9.7 +/- 6.6% in 80 mM [K(+)] stimulated cells. The percentage of mitochondrial membrane closely associated with the SR was correspondingly lower at 10.1 +/- 1.0% during ACh stimulation and 10.8 +/- 0.9% during 80 mM [K(+)] stimulation. During smooth muscle cell stimulation, the SR appears to unwrap from the mitochondria and extend into the cytoplasm while maintaining close contact with the mitochondria over a smaller area. Such static and dynamic components of the close spatial association between the mitochondria and the SR may serve as a structural basis for the selective and efficient Ca(2+) trafficking between the two organelles in TSMC.     

Electromyographic activity and rate of muscle fatigue of the quadriceps femoris during cycling exercise in the severe domain

This study compared the activation pattern and the fatigue rate among the superficial muscles of the quadriceps femoris (QF) during severe cycling exercise. Peak oxygen consumption (VO(2)peak) and maximal accumulated oxygen Deficit (MAOD) were established by 10 well-trained male cyclists (27.5 ± 4.1 years, 71.0 ± 10.3 kg, 173.4 ± 6.6 cm, mean VO(2)peak 56.7 ± 4.4 ml·kg·min(-1), mean MAOD 5.7 ± 1.1 L). Muscle activity (electromyographic [EMG] signals) was obtained during the supramaximal constant workload test (MAOD) and expressed by root mean square (RMS) and median frequency (MF slope). The RMS of the QF, vastus lateralis (VL) and vastus medialis (VM) muscles were significantly higher than at the beginning after 75% of exercise duration, whereas for the rectus femoris (RF), this was observed after 50% of exercise duration (p ≤ 0.05). The slope of the MF was significantly higher in the RF, followed by the VL and VM (-3.13 ± 0.52 vs. -2.61 ± 0.62 vs. -1.81 ±0.56, respectively; p < 0.05). We conclude that RF may play an important role in limiting performance during severe cycling exercise.     

The organization of testicular interstitial tissue and changes in the fine structure of the Leydig cells of European moles (Talpa europaea) throughout the year

Seasonal changes of the testicular interstitial tissue were studied by electron microscopy. During the breeding season in spring, clusters of Leydig cells are surrounded by wide lymphatic sinusoids. In sexually quiescent moles, these sinusoids collapse, and the abundant Leydig cells become closely packed and occupy most of the testis. During sexual activity, the Leydig cells contain abundant smooth endoplasmic reticulum (SER), mitochondria with tubular cristae, and lipid droplets. Some areas of the cytoplasm are occupied exclusively by tubular SER, arranged in parallel. During regression the SER appears tortuous, and large lipid droplets are found in the cytoplasm, although these gradually become smaller. During the long period of sexual quiescence, the size and abundance of Leydig cells and the appearance of SER, lipid droplets and mitochondria were similar to those observed during sexual activity.     

Structural changes in bovine oocytes during final maturation in vivo

On the basis of structural observations bovine oocytes were grouped into four successive classed: 0, those before the luteinizing hormone (LH) surge; 1, those up to 8 h following the LH peak level; 2, those between 8 and 19 h after the LH peak level; and 3, those between 19 h after the LH peak level and ovulation. Oocytes in class 0 had mitochondria located in a generally peripheral position. Interior to the mitochondria were elements of rough endoplasmic reticulum (RER) and numerous membrane-bound vesicles which bore ribosome-like particles on their outer surface. The first visible changesater the LH peak level as seen in class 1 were the formation of the periviteline space with loss of contact between the cumulus cells and the oocyte, and ruffing of the nuclear envelope. These changes were followed b the resumption of meiosis as defined by germinal-vesicle breakdown (GVBD), the disappearance of RER, and the formation fo clusters of mitochondria in association with lipid droplets and elementrs of smooth endolasmic reticulum (SER). The period between 8 and 19 h following LH peak level (class 2) was characterized by intensive clustering of mitochoncria in association with lipid droplets and elements of SER, conversion of lipid, fusion of vesicles, and the appearance of ribosomes in the cytoplasm. During the final stage (class 3), the polar body was extruded, the mitochondria dispersed, and the majority of the organelles became located toward the center of the cell. The relatively organelle-free cortical region contained cortical granules immediately adjacent to the plasma membrane together with aggregates of tubular SER. The structural changes are discussed in the context of follicular steroidogenesis and oocyte developmental competence.     

Morphometric analysis of the pineal complex of the golden hamster over a 24-hour light:dark cycle: I. The superficial pineal in untreated and optically enucleated animals

Morphometric analysis of the superficial pineal gland of intact and blinded golden hamsters was conducted at both the light and electron microscopic level. The volume of the superficial gland was estimated to be 151 X 10(6) micron 3, comprising 90-94% of the total pineal parenchymal tissue. Analysis of structural rhythms in animals maintained under a 14:10 L:D cycle showed significant 24-hr variations in values for pinealocyte nuclei, nucleoli, rough and smooth endoplasmic reticulum, Golgi bodies, dense bodies, and dense-cored vesicles. Peak values for these structures generally occurred at the light:dark interface. These results provide morphological correlates for known rhythmic variations in the synthesis of pineal-gland products. Superficial pineals examined 8 weeks following optic enucleation exhibited a decrease in the volume of pinealocyte nuclei and cytoplasm, while nucleolar size and the amounts of smooth and rough endoplasmic reticulum, Golgi bodies, dense bodies and dense-cored vesicles were enhanced. The latter changes are interpreted as indications of increased synthetic activity by the superficial pineal gland in response to light deprivation.     

Ca2+ regulation in the near-membrane microenvironment in smooth muscle cells

The microenvironment between the plasma membrane and the near-membrane sarcoplasmic reticulum (SR) may play an important role in Ca(2+) regulation in smooth muscle cells. We used a three-dimensional mathematical model of Ca(2+) diffusion and regulation and experimental measurements of SR Ca(2+) uptake and the distribution of the SR in isolated smooth muscle cells to predict the extent that the near-membrane SR could load Ca(2+) after the opening of single plasma membrane Ca(2+) channels. We also modeled the effect of SR uptake on 1), single-channel Ca(2+) transients in the near-membrane space; 2), the association of Ca(2+) with Ca(2+) buffers in this space; and 3), the amount of Ca(2+) reaching the central cytoplasm of the cell. Our results indicate that, although single-channel Ca(2+) transients could increase SR Ca(2+) to a certain extent, SR Ca(2+) uptake is not rapid enough to greatly affect the magnitude of these transients or their spread to the central cytoplasm unless the Ca(2+) uptake rate of the peripheral SR is an order-of-magnitude higher than the mean rate derived from our experiments. Immunofluorescence imaging, however, did not reveal obvious differences in the density of SR Ca(2+) pumps or phospholamban between the peripheral and central SR in smooth muscle cells.