Surface charge potentiates conduction through the cardiac ryanodine receptor channel.

Abstract

A B S T R A C T Single channel currents through cardiac sarcoplasmic reticulum (SR)Ca 2 § release channels were measured in very low levels of current carrier (e.g., 1 mM Ba2+). The hypothesis that surface charge contributes to these anomalously large single channel currents was tested by changing ionic strength and surface charge density. Channel identity and sidedness was pharmacologically determined. At low ionic strength (20 mM Cs+), Cs + conduction in the lumen ~ myoplasm (L--> M) direction was significantly greater than in the reverse direction (301.7 + 92.5 vs 59.8 -38 pS, P < 0.001; mean ~ SD, t test). The Cs + concentration at which conduction reached half saturation was asymmetric (32 vs 222 mM) and voltage independent. At high ionic strength (400 mM Cs+), conduction in both direction saturated at 550 --. 32 pS. Further, neutralization of carboxyl groups on the lumenal side of the channel significantly reduced conduction (333.0 -+ 22.5 vs 216.2 -+ 24.4 pS, P < 0.002). These results indicate that negative surface charge exists near the lumenal mouth of the channel but outside the electric field of the membrane. In vivo, this surface charge may potentiate conduction by increasing the local Ca 2+ concentration and thus act as a preselection filter for this poorly selective channel.

Characterization of the Calcium-release Channel/Ryanodine Receptor from Zebrafish Skeletal Muscle

“…The predominant RyR isoform in mammalian skeletal muscle, RyR1, shows a similar activation by physiologically low Ca 2+ concentrations but a distinct inactivation at high Ca 2+ concentrations (this study; Bezprozvanny et al, 1991, Marks et al, 1989Meissner, 1994). Mammalian RyR2 and RyR3 show a reduced sensitivity for high Ca 2+ concentrations and remain more active under such conditions (Sonnleitner et al, 1998;Holmberg & Williams, 1990;Tu et al, 1994;Valdivia, 1998). The results of the present study show that zebrafish RyRs provide an excellent model system for mammalian physiology: The direct comparison of RyR function in nonmammalian and mammalian skeletal muscle has to be carefully evaluated given the different Ca 2+ dependence of the channels involved.…”

Reversible Block of the Calcium Release Channel/Ryanodine Receptor by Protamine, a Heparin Antidote

“…Such alterations caused by polyanions would lead to an increase in the local negative charge of the receptor complex and thus attract more calcium ions activating the receptor at its calciumregulatory sites (Bezprozvanny et al, 1993). The importance of the local charge around the pore of the RyR has been demonstrated by Tu et al (1994). In that study, the importance of a negative surface charge for the potentiation of conduction and selectivity of the channel was shown.…”

“…They provided evidence for a mechanism in which the binding of polyanions to the RyR would increase the local negative charge of the receptor complex and thus attract more calcium ions to activate the receptor. Additional support for this mechanism is found in the facts that the endogenous RyR has a fixed negative surface charge and that this surface charge potentiates conduction and conveys divalent cation selectivity of the channel (Tu et al, 1994).In the present study, we analyzed the effects of protamine (clupeine) on the RyR. Protamines are a known antidote to heparin (Byun et al, 1999), are highly positively charged, and are rich in basic amino acids (Felix, 1960).…”

Regulation of calcium release by calcium inside the sarcoplasmic reticulum in ventricular myocytes

“…However, it should be noted that because of the progressive saturation of intraluminal Cat buffers (i.e., calsequestrin) during Cat overload, the increase in intraluminal free [Ca 2 +] (and in the gradient across the SR membrane) may be considerably higher than that suggested by changes in the total SR Cat content. On the other hand, currents carried by divalent cations through the Cat release channel are known to saturate when the concentration of the cations on the luminal side is raised to between 2 and 5 mM [25,26]. If the SR free [Ca2+] in normal myocytes is about 1-3 mM, as is estimated for skeletal muscle [27], any further increase in luminal [Ca2+] would not produce a significant change in current amplitude.…”

“…In addition, if luminal Cat had access to the cytoplasmic activation site, then Mgt and Bat, both of which concurrently inhibit Cat activation of the channel and have high relative permeability, would be expected to reduce channel activity when added to the luminal side. Neither Mgt or Bat at high luminal concentrations affect channel gating [25,26]. Regardless of the exact mechanism, the ability to respond to elevations of luminal [Ca2+] appears to be a fundamental property of the RyR channel, which may account for, or contribute to, the potentiation of Cat+ release at increased levels of SR Caz+ load.…”