  The function of many membrane-enclosed intracellular structures relies on
release of diffusing particles that exit through narrow pores or channels in
the membrane. The rate of release varies with pore size, density, and length of
the channel. We propose a simple approximate model, validated with stochastic
simulations, for estimating the effective release rate from cylinders, and
other simple-shaped domains, as a function of channel parameters. The results
demonstrate that, for very small pores, a low density of channels scattered
over the boundary is sufficient to achieve substantial rates of particle
release. Furthermore, we show that increasing the length of passive channels
will both reduce release rates and lead to a less steep dependence on channel
density. Our results are compared to previously-measured local calcium release
rates from tubules of the endoplasmic reticulum, providing an estimate of the
relevant channel density responsible for the observed calcium efflux.
