Calcium transients and the effect of a photolytically released calcium chelator during electrically induced contractions in rabbit rectococcygeus smooth muscle

The small GTPase Rac1 is required for smooth muscle contraction

The role of the small GTP-binding protein Rac1 in smooth muscle contraction was examined using small molecule inhibitors (EHT1864, NSC23766) and a novel smooth muscle-specific, conditional, Rac1 knockout mouse strain. EHT1864, which affects nucleotide binding and inhibits Rac1 activity, concentration-dependently inhibited the contractile responses induced by several different modes of activation (high-K+, phenylephrine, carbachol and protein kinase C activation by phorbol-12,13-dibutyrate) in several different visceral (urinary bladder, ileum) and vascular (mesenteric artery, saphenous artery, aorta) smooth muscle tissues. This contractile inhibition was associated with inhibition of the Ca2+ transient. Knockout of Rac1 (with a 50% loss of Rac1 protein) lowered active stress in the urinary bladder and the saphenous artery consistent with a role of Rac1 in facilitating smooth muscle contraction. NSC23766, which blocks interaction between Rac1 and some guanine nucleotide exchange factors, specifically inhibited the α1 receptor responses (phenylephrine) in vascular tissues and potentiated prostaglandin F2α and thromboxane (U46619) receptor responses. The latter potentiating effect occurred at lowered intracellular [Ca2+]. These results show that Rac1 activity is required for active contraction in smooth muscle, probably via enabling an adequate Ca2+ transient. At the same time, specific agonists recruit Rac1 signalling via upstream modulators, resulting in either a potentiation of contraction via Ca2+ mobilization (α1 receptor stimulation) or an attenuated contraction via inhibition of Ca2+ sensitization (prostaglandin and thromboxane receptors).

Intracellular calcium in hypertrophic smooth muscle from rat urinary bladder

OBJECTIVE

To explore whether infravesical outlet obstruction is associated with alterations in calcium activation of detrusor smooth muscle.

MATERIAL AND METHODS

Outlet obstruction was created by partial ligature of the urethra in female rats. Western blotting was performed using an antibody against the cytoplasmatic region of the alpha1c subunit of the L-type Ca2+ channel. Intracellular calcium was measured using Fura-2 in detrusors that had been obstructed for 10 days and activated by high K+ concentrations at different extracellular Ca2+ concentrations. The rate of force development after rapid opening of L-type Ca2+ channels was measured in contractions initiated by flash photolysis of nifedipine in Ca2(+)-containing depolarizing solution.

RESULTS

Bladder weight increased from 62 +/- 3 to 254 +/- 43 mg after 10 days of obstruction. Expression of the alpha1c subunit increased after 3 days and continued to increase until it was about fourfold greater after 10 days; however, it had not increased further at 6 weeks. This change was reversible after removal of obstruction. Activation with K+ produced a stable force at different extracellular Ca2+ concentrations, with no difference in response between controls and rats that had been obstructed for 10 days. Intracellular Ca2+ concentrations were lower in the obstructed group, showing that the calcium sensitivity of the contraction force had increased. The delay between the opening of L-type channels and the onset of contraction was longer in obstructed detrusors.

CONCLUSIONS

Growth of detrusor muscle following obstruction is accompanied by attenuated calcium transients following activation, despite upregulation of L-type Ca2+ channels. The Ca2+ sensitivity of contraction was increased in obstructed detrusors. We suggest that the decreased surface: volume ratio in hypertrophic smooth muscle cells is partly involved in the lowered Ca2+ transients. The increases in L-type calcium channels and in calcium sensitivity may be compensatory mechanisms.

Shortening induced effects on force (re)development in pig urinary smooth muscle

INTRODUCTION

When muscle is allowed to shorten during an active contraction, the maximum force that redevelops after shortening is smaller than the isometric force at the same muscle length without prior shortening. We studied the course of force redevelopment after shortening in smooth muscle to unravel the mechanism responsible for this deactivation.

METHOD

In a first series of measurements the shortening velocity was varied resulting in different shortening amplitudes. In a second series, the duration of stimulation before shortening (shortening delay) was varied. In a third series, the stimulation was interrupted for a certain duration immediately after shortening. Force, muscle length and stimulation were continuously recorded. Time constants were calculated to describe the rate of force development before and after shortening.

RESULTS

With increasing shortening amplitude and with increasing shortening delay, force redevelopment decreased. Redevelopment increased with an increase in the interruption time. After stimulus interruption force redeveloped mono-exponentially with a time constant similar to that of isometric contractions (approximately 3s). Without the interruption of stimulation, the redevelopment of force immediately after shortening was best described by two time constants; one similar to and one about 3-5 times faster than the isometric time constant.

DISCUSSION

Force (re)development is caused by a cascade of events leading to the cycling of cross-bridges. In smooth muscle, isometric force development is described by a time constant of about 3s. Force redevelopment immediately after shortening involves a second process which takes place at a faster rate (time constant about 1s). We assume that this process is faster due to the immediate availability of cytoplasmic calcium released during active shortening. Deactivation presumably is caused by disorganization of filaments during shortening.

Fluorescence lifetime microscopy of the Na+ indicator Sodium Green in HeLa cells

This study investigates the usefulness of lifetime measurements of Sodium Green for evaluating intracellular Na+ concentration ([Na+]i) in HeLa cells. Frequency-domain lifetime measurements are performed in HeLa cells and in different buffer solutions (with and without K+ and bovine serum albumin). In all cases, the fluorescence decays of Sodium Green are multiexponential, with decay times independent of [Na+]. Three relaxation times are found in the various buffer solutions. Binding of the indicator to albumin results in an increase in the long and intermediate decay times. For Sodium Green inside HeLa cells, the intensity decay can be approximated by a biexponential. The ratio of the fractional intensity of the long decay time (tau2 = 2.4 +/- 0.2 ns) to that of the short component (tau1 = 0.4 +/- 0.1 ns) increases with [Na+]i. The changes in fluorescence decay with [Na+] are significantly less pronounced in cells as compared with the buffer solutions. Similar values for the resting [Na+]i were estimated from lifetime measurements of Sodium Green and from ratiometric measurements using SBFI. Alternatively, [Na+]i can be monitored by measuring only the phase angle at the modulation frequency of 160 MHz. The usefulness of this latter approach is demonstrated by following the changes in [Na+]i induced by reversible inhibition of the Na+/K+ pump.

Fluorescence lifetime microscopy of the sodium indicator sodium-binding benzofuran isophthalate in HeLa cells

The behavior of the sodium indicator sodium-binding benzofuran isophthalate (SBFI) is investigated in HeLa cells by time-resolved fluorescence microscopy. The fluorescence relaxation of SBFI in HeLa cells can be described by a triexponential for intracellular sodium concentration ([Na(+)](i)) between 0 and 90 mM. Changes in [Na(+)](i) affect neither the fluorescence relaxation times (0.21, 0. 60, and 2.7 ns) nor the average decay time (2.2 ns). The preexponential factor of the shortest decay time is negative. However, the ratio of the fluorescence excitation signal at 340 nm to that at 380 nm increases with [Na(+)](i). To elucidate the behavior of SBFI in cells, experiments are performed on SBFI in buffer at various concentrations of sodium, potassium, and bovine serum albumin (BSA) and at various viscosities. The fluorescence decay is triexponential only in the presence of BSA. The relaxation times are independent of [Na(+)] and [BSA]. The preexponential factor of the shortest decay time is negative from a certain [BSA] on, which depends on [Na(+)]. The data indicate that interactions with intracellular components rather than microviscosity influence the SBFI behavior in cells. A model is suggested in which the fluorescence intensities are mainly determined by the signals from the Na(+) subsetSBFI and SBFI subsetprotein complexes.

Kinetics of contraction in depolarized smooth muscle from guinea-pig taenia coli after photodestruction of nifedipine

1. The time course and kinetics of force development following activation by opening of L-type Ca2+ channels was investigated using photodestruction of the Ca2+ channel blocker nifedipine in smooth muscle from the guinea-pig taenia coli. 2. In muscles activated using high K+ and Ca2+ and subsequently inhibited with nifedipine, photodestruction of the drug using a strong ultraviolet light flash initiated a rapid contraction. The force initiated by photodestruction of nifedipine reached near-maximal levels. This procedure eliminates diffusional delays and can thus be used to investigate the kinetics of depolarization-induced contractions. 3. The rate of force development of contractions initiated by photodestruction of nifedipine was slower than that observed in maximally thiophosphorylated skinned fibres. This suggests the rate of force development is limited by activation steps in the activation cascade prior to the force generation of the cross-bridge system. 4. The rate of force development and the plateau force were dependent on the extracellular [CaCl2] suggesting that the intracellular [Ca2+] determines the rate of phosphorylation and force development. The delay between illumination and increase in force was about 300 ms. The delay was similar at low and high extracellular [CaCl2] indicating that buffering by superficial sarcoplasmatic reticulum does not introduce a delay in force development following activation of Ca2+ channels in this muscle.