Thermal sensitivity of escape response performance by the scallop Placopecten magellanicus: Impact of environmental history

To examine the impact of environmental history on the thermal sensitivity of escape response performance in juvenile giant scallops, Placopecten magellanicus, we compared animals sampled in late May, when water temperatures and day length were increasing, to animals sampled in late September, when water temperature and day length were decreasing. Habitat temperature was approximately 12 °C at both sampling times. For May scallops, performance was better at 6 than at 12 or 18 °C whereas September scallops performed better at 6 and 12 °C than at 19 °C. Regardless of environmental history, the rate of phasic contractions consistently declined at 18–19 °C, due to a decrease in the number of phasic contractions. Force measurements during escape responses of May scallops showed that phasic force production and the minimal interval between contractions changed little with temperature, whereas the minimum and mean durations of phasic contractions decreased as temperature rose. Phasic contraction rate in the first series increased with temperature. Reliance upon tonic contractions was higher in scallops tested at 18 °C than in those tested at 6 °C. Environmental history, more than habitat temperature at the time of sampling, seems to set the thermal sensitivity of phasic contraction rate in P. magellanicus. Phasic force production did not change within the thermal range tested.     

Tonic contractions allow metabolic recuperation of the adductor muscle during escape responses of giant scallop Placopecten magellanicus

To escape from starfish predators, giant scallops, Placopecten magellanicus, swim using series of strong phasic contractions interrupted by tonic contractions. To investigate whether these tonic contractions allow metabolic recuperation of the adductor muscle, we sampled scallops at rest (Control), after an initial series of phasic contractions (Phasic) and after 1 min of tonic contraction following their initial phasic contractions (Phasic + Tonic) and compared muscle levels of phosphoarginine, adenylate nucleotides (ATP, ADP and AMP) and adenylate energy charge (AEC). Scallops in the two active groups did not differ in the numbers of phasic contractions or the mean phasic force production. Phosphoarginine concentrations in the adductor muscle decreased with phasic activity and remained low after 1 min of tonic contraction. ATP and ADP and total adenylate levels did not differ between the three groups, but AMP levels were higher in the scallops sampled after phasic contractions than in control scallops. The AEC was reduced by phasic contractions but returned to control levels after 1 min of tonic contraction. A significant negative correlation between AEC and the number of claps in the Phasic group disappeared in the Phasic + Tonic group. Thus, tonic contractions following phasic contractions allow partial metabolic recovery of the adductor muscle by returning AEC to control levels. However, phosphoarginine levels did not recover during tonic contractions, and a negative correlation between the number of claps and phosphoarginine levels remained in the Phasic + Tonic group. By interspersing tonic contractions between series of phasic contractions, scallops improved muscle energetic status, which should help maintain phasic force production during the remainder of the escape response.     

Sensitization does not alter phasic and tonic responses to carbachol in isolated epithelium-free guinea-pig trachea

An alteration in the handling of Ca2+ has been proposed as the pathogenic mechanism underlying the airway smooth muscle hyperresponsiveness of asthma. The present study tested the hypothesis that the altered responsiveness of receptor operated contraction to carbachol in allergic asthma results from a change in the phasic or tonic components. Using a kinetic approach, the phasic and tonic responses to 10 microM carbachol were quantitated in isolated epithelium-free trachea 21 days after guinea-pigs were sensitized with ovalbumin and aluminum hydroxide (as adjuvant) to generate preferentially IgE-like antibodies. Sensitization was confirmed by challenge of the isolated trachea with ovalbumin. The steady-state and kinetic characteristics of the phasic and tonic responses were the same from sensitized animals and animals treated with saline and aluminum hydroxide (control) and before and after challenge of the trachea from both groups of animals. The present results demonstrate that immunologic sensitization and challenge do not appear to elicit a defect in the phasic or tonic responses of receptor mediated contractions in airway smooth muscle and suggest there is no alteration in the handling of Ca2+ in smooth muscle from sensitized and challenged guinea-pig trachea.     

Seasonal changes in the effects of arginine vasotocin and stretch on Anolis uterine contractions in vitro

We determined the effects of acute stretch on spontaneous and arginine vasotocin (AVT)-driven contractions of the Anolis carolinensis uterus in vitro. Whole uteri from reproductively inactive females (October) were placed in a bath of oxygenated 32 degrees C Anolis "Ringer's." Two initial tensions were utilized, 1.5 g or 15 g, the latter being an estimate of the tension on the wall of a uterine compartment. Uteri were then exposed to either saline or AVT (50 ng/ml), and spontaneous or AVT-driven contractions were recorded for 20 min with the use of a strain gauge and physiograph. A similar experiment was performed on uteri from reproductively active females in the summer (June). Our results indicate that the effects of acute stretch and AVT on uterine contractility were qualitatively similar in summer and fall. That is, AVT induced a tonic contraction; stretch decreased the duration of the tonic contraction; the saline-treated uteri exhibited spontaneous rhythmic contractions; AVT increased the amplitude of the rhythmic contractions, but only at the lower tension; there were no effects of AVT on the timing (contraction interval, duration, rest interval) of the rhythmic contractions; and stretch increased the frequency of the rhythmic contractions. Season greatly influenced the magnitude of these contractile phenomena. Uteri tested during the breeding season exhibited greater distensibility, an increase in the amplitude and duration of the AVT-driven tonic contraction, and an increase in the frequency of both spontaneous and AVT-driven rhythmic contractions because of a decrease in both contraction duration and rest interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lateral giant fibre activation of exopodite motor neurones in the crayfish tailfan

The uropods of decapod crustaceans play a major role in the production of thrust during escape swimming. Here we analyse the output connections of a pair of giant interneurones, that mediate and co-ordinate swimming tail flips, on motor neurones that control the exopodite muscles of the uropods. The lateral giants make short latency output connections with phasic uropod motor neurones, including the productor, the lateral abductor and adductor exopodite motor neurones that we have identified both physiologically and anatomically. On the other hand, tonic motor neurones, including the ventral abductor and reductor exopodite motor neurones, receive no input from the lateral giants. We show that there is no simple reciprocal activation of the phasic opener (lateral abductor) and closer (adductor) motor neurones of the exopodite, but instead both phasic motor neurones are activated in parallel with the productor motor neurone during a tail flip. Our results show that the neuronal pathways activating the tonic and phasic motor neurones of the exopodite are apparently independent, with phasic motor neurones being activated during escape movements and tonic motor neurones being activated during slow postural movements.     

Calcium-independent contraction and sensitization of airway smooth muscle by p21-activated protein kinase

In Triton-skinned phasic ileal smooth muscle, constitutively active recombinant p21-activated kinase (PAK3) has been shown to induce Ca(2+)-independent contraction, which is accompanied by phosphorylation of caldesmon and desmin (Van Eyk JE, Arrell DK, Foster DB, Strauss JD, Heinonen TY, Furmaniak-Kazmierczak E, Cote GP, and Mak AS. J Biol Chem 273: 23433-23439, 1998). In the present study, we investigated whether PAK has a broad impact on smooth muscle in general by testing the hypothesis that PAK induces Ca(2+)-independent contractions and/or Ca(2+) sensitization in tonic airway smooth muscle and that the process is mediated via phosphorylation of caldesmon. In the absence of Ca(2+) (pCa > 9), constitutively active glutathione-S-transferase-murine PAK3 (GST-mPAK3) caused force generation of Triton-skinned canine tracheal smooth muscle (TSM) fibers to approximately 40% of the maximal force generated by Ca(2+) at pCa 4.4. In addition, GST-mPAK3 enhanced Ca(2+) sensitivity of contraction by increasing force generation by 80% at intermediate Ca(2+) concentrations (pCa 6.2), whereas it had no effect at pCa 4.4. Catalytically inactive GST-mPAK3(K297R) had no effect on force production. Using antibody against one of the PAK-phosphorylated sites (Ser(657)) on caldesmon, we showed that a basal level of phosphorylation of caldesmon occurs at this site in skinned TSM and that PAK-induced contraction was accompanied by a significant increase in the level of phosphorylation. Western blot analyses show that PAK1 is the predominant PAK isoform expressed in murine, rat, canine, and porcine TSM. We conclude that PAK causes Ca(2+)-independent contractions and produces Ca(2+) sensitization of skinned phasic and tonic smooth muscle, which involves an incremental increase in caldesmon phosphorylation.     

Ultrastructure of a molluscan smooth muscle during phasic contractions and in the relaxed state

Tension and X-ray diffraction patterns are not always correlated in the smooth anterior retractor muscle (ABRM) of Mytilus edulis. The muscle produces equatorial intensity profiles of X-ray diffraction patterns corresponding to either a relaxed or a contracted structure. During phasic contractions, comprising a contracted as well a a relaxed phase, the diffracted intensity on the equator at 0.003 A^?1 changes within the first 10s after onset of stimulation. The tension reaches a maximum after about the same time. The time dependence of this intensity change during phasic contraction has been measured. It shows that the tension decays within 10s, but the relaxed structure needs 30–40 s to reestablish. There is no difference between the observed intensities from the tonic and phasic contracted states. Inactivated muscles with minimum tension, normally termed relaxed, can have either a “contracted” or a relaxed structure.     

The effect of D600 on tonic tension, Na+ inward current, and Na+-Ca2+ exchange in frog heart

Preparations of frog atrial muscle were stimulated at 0.33 Hz under voltage clamp, and the resulting membrane currents and the twitch contractions (phasic and tonic components) were recorded in presence or absence of D600. It has been suggested earlier that the tonic contractions are regulated by an electrogenic Na+-Ca2+ exchange, while the phasic contractions are closely related to the calcium inward current (Isi). In this study we investigated the effect of D600 on (i) the tonic contractions elicited by long depolarizing pulses of high amplitude and (ii) the tonic contractions increased by veratrine and resulting in a positive inotropic effect (PIE). While 1 microM D600 reduced Isi and the corresponding phasic contractions to less than 30% of their initial values within 5 min, the inhibitory effect of D600 on tonic contractions developed more slowly or higher concentrations of D600 were needed to achieve similar levels of inhibition within the same time. Furthermore, applications of 5-50 microM D600 inhibited the veratrine-induced increase in INa and in tonic contractions, and both of these effects again fully developed within a few minutes of D600 being removed. The results demonstrate that D600 inhibits not only Isi and phasic contractions, but it also decreases the tonic contractions in frog heart. The effect on the tonic component is associated with inhibition of the tetrodotoxin-sensitive Na+ inward current, and the results are interpreted as an effect of D600 on the electrogenic Na+-Ca2+ exchange. These additional effects of D600 should be considered when using this drug as the "specific" calcium channel blocker.     

Oxytocin-induced phasic and tonic contractions are modulated by the contractile machinery rather than the quantity of oxytocin receptor

To investigate the relationship between the oxytocin (OT) receptor (OTR) quantity and the contractile features systematically, we measured the mRNA expression levels of OTR and L-type Ca(2+) channel alpha(1C)-subunit (alpha(1C)) and examined the regulatory mechanisms of OT-induced phasic or tonic contractions of the longitudinal smooth muscles in mouse uteri. The mRNA expression of OTR in 19.0 G (19.0 days of gestation) was greater than those in nonpregnant phases, and that of alpha(1C) in estrus and 19.0 G was higher than in diestrus. OT-induced contractions sparsely occurred in diestrus. The OT-induced all-or-none-type phasic contractions at low concentrations were abolished by verapamil in both estrus and 19.0 G. OT-induced tonic contractions had similar pD(2) values in both estrus and 19.0 G. However, the magnitude in 19.0 G was much greater than that in estrus. The large tonic contractions also occurred in PGF(2alpha) receptor (FP) knockout mice in 19.0 G despite a small amount of OTR. Verapamil and Y-27632 partially inhibited the tonic contractions in 19.0 G. Cyclopiazonic acid-induced tonic contractions were reciprocally decreased with the increase in the OT-induced ones in 19.0 G. These results indicate that the phasic contractions are dependent on alpha(1C). The tonic contractions in 19.0 G are dependent on both Ca(2+) influxes via L-type Ca(2+) channels and store-operated Ca(2+) channels, and the force is augmented by the Rho signal pathway, which increases the Ca(2+) sensitivity. Thus the uterine contractions are mainly controlled by the modification of contractile signal machinery rather than simply by the OTR quantity.     

Effect of reproduction on escape responses and muscle metabolic capacities in the scallop Chlamys islandica Müller 1776

In scallops, gametogenesis leads to mobilization of glycogen and proteins from the adductor muscle towards the gonad. This mobilization is likely to diminish the metabolic capacities of the adductor muscle and thereby the scallops' escape response. We examined the escape response in terms of number of valve claps until exhaustion, rate of clapping and the recovery during and after valve closure in adult scallops, Chlamys islandica, sampled at different stages in the reproductive cycle (immature, mature, before and after spawning). In parallel, we measured muscle glycogen, protein and phosphoarginine contents, the oxidative capacity of mitochondria isolated from the adductor muscle and levels of muscle enzymes which are active during exercise and recovery. The number of claps (24-26), rate of clapping ( approximately 13 clapsmin(-1)) and phosphoarginine and arginine kinase levels were similar during the different reproductive stages. All immature scallops responded to restimulation immediately after opening their valves, while only 62% of mature, 82% of prespawned and 38% of spawned scallops responded. Immature animals completely recovered their initial swimming capacity within 4 h of opening their valves, but mature, prespawned and spawned scallops needed 18, 12 and 18 h, respectively. Overall phasic adductor muscle from mature, prespawned and spawned animals showed decreased glycogen phosphorylase, phosphofructokinase, pyruvate kinase (except for prespawned), octopine dehydrogenase and citrate synthase levels, a deterioration of the oxidative capacity of mitochondria and a marked decrease in glycogen content compared to immature scallops. Therefore, during gonadal maturation and spawning, C. islandica did not change its clapping capacity, but slowed its recuperation from exhausting burst exercise, both during and after valve closure, likely due to the decreased metabolic capacity of the adductor muscle.     

Effect of He-Ne laser irradiation on the functional activity of smooth muscle cells in the portal vein]

Irradiation of the rat portal vein's fragments with the He-Ne laser for 3, 5 and 10 minutes reduced the fragments tone by half. Frequency of phasic and tonic contractions did not change, their amplitude, however, increased by neatly 40% as compared with the initial level. The NO synthase blocker N-nitro-L-arginine administered prior to the irradiation had no effect on the above parameters. The data obtained suggest that the decrease of the vessel tone is due to production of the EDRF and cGMP. The increase in the amplitude of phasic and tonic contractions of the vein's smooth muscle cells is associated with an increased Ca++ entry in each contraction cycle.     

Neurogenic contractions of the rat tail artery under isobaric conditions: effect of transmural pressure and function of the endothelium]

In stimulation of the rat nerve with a modulated sine pattern, an increase in the modulating frequency from 0.03 to 0.15 Hz diminished the latency between the stimulating signals and changes in the vessel resistance as well as the amplitude of the flow oscillations, but did not affect tonic contractions of the vessel. A reduction of transmural pressure from 80 to 40 mm Hg increased both the tonic and the phasic components of the vessel contraction. Following the endothelium removal no change in the response latency occurred. The data obtained suggest that, during a rhythmic neurogenic influence, the vascular endothelium may work as an "amplifier" of the vessel's phasic contractions.     

Messenger molecules of the phospholipase signaling system have dual effects on vascular smooth muscle contraction

Background and methods. In order to investigate the role of phospholipases and their immediately derived messengers in agonist-induced contraction of portal vein smooth muscle, we used the addition in the organ bath of exogenous molecules such as: phospholipases C, A(2), and D, diacylglycerol, arachidonic acid, phosphatidic acid, choline. We also used substances modulating activity of downstream molecules like protein kinase C, phosphatidic acid phosphohydrolase, or cyclooxygenase. Results. a) Exogenous phospholipases C or A(2), respectively, induced small agonist-like contractions, while exogenous phospholipase D did not. Moreover, phospholipase D inhibited spontaneous contractions. However, when added during noradrenaline-induced plateau, phospholipase D shortly potentiated it. b) The protein kinase C activator, phorbol dibutyrate potentiated both the exogenous phospholipase C-induced contraction and the noradrenaline-induced plateau, while the protein kinase C inhibitor 1-(-5-isoquinolinesulfonyl)-2-methyl-piperazine relaxed the plateau. c) When added before noradrenaline, indomethacin inhibited both phasic and tonic contractions, but when added during the tonic contraction shortly potentiated it. Arachidonic acid strongly potentiated both spontaneous and noradrenaline-induced contractions, irrespective of the moment of its addition. d) In contrast, phosphatidic acid inhibited spontaneous contractile activity, nevertheless it was occasionally capable of inducing small contractions, and when repetitively added during the agonist-induced tonic contraction, produced short potentiations of the plateau. Pretreatment with propranolol inhibited noradrenaline-induced contractions and further addition of phosphatidic acid augmented this inhibition. Choline augmented the duration and amplitude of noradrenaline-induced tonic contraction and final contractile oscillations. Conclusions. These data suggest that messengers produced by phospholipase C and phospholipase A(2) contribute to achieve the onset and maintenance of contraction, while phospholipase D-yielded messengers appear to provide a delayed "on/off switch" that ultimately brings relaxation.     

Morphofunctional and histochemical characteristics of the hindlimb muscles of the Rana temporaria in ontogeny

Mixed muscles of adult frogs respond to the increase in external potassium and to Ach by polyphasic contracture which is due to asynchronous activity of various groups of muscle fibers (fast phasic, intermediate and tonic ones). In the developing in vivo hindlimb muscles, the predominance of phasic contractile response and relatively weak tonic one were noted. In contrast to definitive muscles, in which maximum potassium and acetylcholine contractures are identical, growing muscles produce weak contractile reaction to Ach. Ach sensitivity of the developing muscles (as revealed by the contracture) is lower than in the definitive ones. Histochemical (studies on the lipid content and the activity of succinate dehydrogenase) and morphometric (the ratio of muscle fibers of different types at different stages of development, comparison of their diameters, relative size of tonic bundle, etc.) studies indicate that the development of morphological substrate for tonic contractions (tonic and intermediate muscle fibers) takes place at a lower rate as compared to the development of the substrate for phasic contractions. However, histochemically tonic fibers may be revealed already at the stage of myotubes.     

Immune responses of the scallop Chlamys farreri after air exposure to different temperatures

The present study examined the influence of air exposure at different temperatures: a common perturbation associated with aquaculture handling practices, on immune responses in zhikong scallop Chlamys farreri. Scallops were exposed to air for 2 h, 6 h, 12 h and 24 h at 5 °C, 17 °C and 25 °C respectively. Thereafter, a recovery period of 24 h at 17 °C was applied. Haemocyte mortality, phagocytosis and reactive oxygen species (ROS) production of haemocytes, acid phosphatase (ACP) and superoxide dismutase (SOD) activity in haemocyte lysates were chosen as immunomarkers of anoxic stress. The results showed that an increase of haemocyte mortality and a decrease of phagocytosis and ACP activity were observed after 2 h of air exposure for all temperatures tested. Moreover, a significant increase of ROS production occurred following 2 h of air exposure at 25 °C and 24 h of air exposure at 17 °C. Significant differences were also observed in haemocyte mortality, percentage of phagocytic cells and ACP and SOD activity depending on the temperature of air exposure. Finally, after 24 h of recovery at 17 °C, percentage of phagocytic haemocytes and ACP activity did not return to initial values. ROS production was significantly higher than before the recovery period and initial values for scallops subjected to air exposure at 5 °C. In our study, scallops showed a relative low anoxia tolerance under a high temperature. All the scallops air exposed to 25 °C died after the 6 h sampling. In conclusion, air exposure associated to aquaculture practices was demonstrated to strongly affect functional immune activities of scallop haemocytes, and high temperature air exposure caused reduced survival of scallops.     

Caveolae and calcium handling, a review and a hypothesis

Caveolae are associated with molecules crucial for calcium handling. This review considers the roles of caveolae in calcium handling for smooth muscle and interstitial cells of Cajal (ICC). Structural studies showed that the plasma membrane calcium pump (PMCA), a sodium-calcium exchanger (NCX1), and a myogenic nNOS appear to be colocalized with caveolin 1, the main constituent of these caveolae. Voltage dependent calcium channels (VDCC) are associated but not co-localized with caveolin 1, as are proteins of the peripheral sarcoplasmic reticulum (SR) such as calreticulin. Only the nNOS is absent from caveolin 1 knockout animals. Functional studies in calcium free media suggest that a source of calcium in tonic smooth muscles exists, partly sequestered from extracellular EGTA. This source supported sustained contractions to carbachol using VDCC and dependent on activity of the SERCA pump. This source is postulated to be caveolae, near peripheral SR. New evidence, presented here, suggests that a similar source exists in phasic smooth muscle of the intestine and its ICC. These results suggest that caveolae and peripheral SR are a functional unit recycling calcium through VDCC and controlling its local concentration. Calcium handling molecules associated with caveolae in smooth muscle and ICC were identified and their possible functions also reviewed.     

Sprouting of substance P-expressing primary afferent central terminals and spinal micturition reflex NK1 receptor dependence after spinal cord injury

The primary afferent neurotransmitter triggering the spinal micturition reflex after complete spinal cord injury (SCI) in the rat is unknown. Substance P detected immunohistochemically in the sacral parasympathetic nucleus was significantly higher in 12 SCI rats than in 12 spinally intact rats (P = 0.008), suggesting substance P as a plausible candidate for the primary afferent neurotransmitter. The effects of the tachykinin NK1 receptor antagonist L-733060 on the spinal micturition reflex were then determined by performing conscious cystometry in an additional 14 intact rats and 14 SCI rats with L-733060 (0.1-100 microg) administered intrathecally at L6-S1. L-733060 was without effect in intact rats, but blocked the spinal micturition reflex in 10 of 14 SCI rats and increased the intermicturition interval in 2 of 4 others at doses ranging from 10 to 100 microg. Both phasic and nonphasic voiding contractions, differentiated according to the presence of phasic external urethral sphincter (EUS) activity, were present in most SCI rats. Both types of contractions were blocked by high doses of L-733060. Interestingly, there was a relative decline in phasic voiding contractions at high doses as well as a decline in contraction amplitude in nonphasic voiding contractions. In other respects, cystometric variables were largely unaffected in either spinally intact or SCI rats. L-733060 did not affect tonic EUS activity at any dose except when the spinal micturition reflex was blocked and tonic activity was consequently lost. These experiments show that tachykinin action at spinal NK1 receptors plays a major role in the spinal micturition reflex in SCI rats.     

Changes of nucleotide contents and of energy charge induced by contraction, catch and relaxation in smooth molluscan muscle fibres. An analysis using reversed-phase ion-pair high-performance liquid chromatography

1. The content of 14 different nucleotides, including cAMP, in isolated muscle fibres of the anterior byssus retractor muscle of Mytilus edulis was analysed. The nucleotide levels were determined after the muscle fibres performed phasic, tonic, or tetanic contractions and after serotonin-induced relaxation of tonic contraction. 2. Isolated resting muscle fibres revealed a lower energy charge than freshly-dissected ones. 3. During active force development adenosine energy charge decreased to stay at the same low level during catch, tetanus and serotonin-induced relaxation of catch respectively. 4. The energy charges of the guanosine, uridine and cytidine systems did not show changes parallel to the adenosine system. 5. The levels of cyclic AMP were only changed under the influence of serotonin.     

The effects of 2-aminoethoxydiphenyl borate, a novel inositol 1,4, 5-trisphosphate receptor modulator on myometrial contractions.

These studies were performed to evaluate the effect of 2-aminoethoxydiphenyl borate (2-APB), a novel membrane-permeable inositol 1,4,5-trisphosphate-receptor inhibitor on agonist-induced, spontaneous, and KCl-stimulated in vitro myometrial contractions. 2-APB significantly inhibited spontaneous myometrial contractions as well as phasic contractions induced by various uterotonic agonists. Confiriming its effects on intracellular calcium release, 2-APB inhibited phasic contractions in the absence of extracellular calcium. 2-APB had little effect on the tonic response to KCl stimulation, implicating its insignificant effect on voltage-gated calcium channels. The inhibitory effect of 2-APB on phasic contractions was completely reversed by washout. In summary, 2-APB effectively penetrated uterine tissue and significantly inhibited myometrial events previously shown to be mediated through activation of the PI-signaling pathway.     

Voltage dependent activation of tonic contraction in cardiac myocytes.

Contractions of isolated single myocytes of guinea pig heart stimulated by rectangular depolarizing pulses consist of a phasic component and a voltage dependent tonic component. In this study we analyzed the mechanism of activation of the graded, sustained contractions elicited by slow ramp depolarization and their relation to the components of contractions elicited by rectangular depolarizing pulses. Experiments were performed at 37 degrees C in ventricular myocytes of guinea pig heart. Voltage-clamped myocytes were stimulated by the pulses from the holding potential of -40 to +5 mV or by ramp depolarization shifting voltage within this range within 6 s. [Ca2+]i was monitored as fluorescence of Indo 1-AM and contractions were recorded with the TV edge-tracking system. Myocytes responded to the ramp depolarization between -25 and -6 mV by the slow, sustained increase in [Ca2+]i and shortening, the maximal amplitude of which was in each cell similar to that of the tonic component of Ca2+ transient and contraction. The contractile responses to ramp depolarization were blocked by 200 microM ryanodine and Ca2+-free solution, but were not blocked by 20 microM nifedipine or 100-200 microM Cd2+ and potentiated by 5 mM Ni2+. The responses to ramp depolarization were with this respect similar to the tonic but not to the phasic component of contraction: both components were blocked by 200 microM ryanodine, and were not blocked by Cd2+ or Ni2+ despite complete inhibition of the phasic Ca2+ current. However, the phasic component but not the tonic component of contraction in cells superfused with Ni2+ was inhibited by nifedipine. Both components of contraction were inhibited by Ca2+-free solution superfused 15 s prior to stimulation. CONCLUSIONS: In myocytes of guinea pig heart the contractile response to ramp depolarization is equivalent to the tonic component of contraction. It is activated by Ca2+ released from the sarcoplasmic reticulum by the ryanodine receptors. Their activation and inactivation is voltage dependent and it does not depend on the Ca2+ influx by the Ca2+ channels or reverse mode Na+/Ca2+ exchange, however, it may depend on Ca2+ influx by some other, not yet defined route.