Ionic diffusion in transverse tubules of cardiac ventricular myocytes

Abstract

We have estimated the rate of diffusion of calcium ions in the transverse tubules of isolated cardiocytes by recording changes in peak calcium current ( I Ca) caused by rapid changes of the extracellular calcium concentration ([Ca]o) at various intervals just preceding activation of I Ca. Isolated ventricular cells of guinea pig heart and atrial cells from rabbit heart were voltage-clamped (whole cell patch), superfused at a high flow rate, and stimulated continuously with depolarizing pulses (0.5 Hz, 200- or 20-ms pulses from a holding potential of −45 or −75 mV to 0 mV). In ventricular cells, the change in peak I Ca following a sudden change of [Ca]oincreased rapidly as the delay between the solution change and depolarization was increased, up to a delay of ∼75 ms [time constant (τ) ≈ 20 ms, 30–40% of total current change), and then increased more slowly (τ ≈ 200 ms, 60–70% of total current change); 400–500 ms were needed to achieve 90% of the total current increase. In atrial cells, a clear separation into two phases was not possible and 90% of the current change occurred within 85 ms. The slow phase of current change, which was unique to the ventricular cells, presumably reflects the slow equilibration of ions between the bulk perfusate and the lumina of the transverse tubules. If the lengths of the transverse tubules were equal to the cell thickness, the slow rate of change of current would be consistent with an apparent diffusion coefficient for calcium ions of 0.95 × 10−6cm2/s, considerably smaller than the value in bulk solution (7.9 × 10−6cm2/s). Most likely, this discrepancy is due to a high degree of tortuosity in the transverse tubular system in guinea pig ventricular cells or possibly to ion binding sites within the tubular membranes and glycocalyx.

Transmission of Information From Cardiac Dihydropyridine Receptor to Ryanodine Receptor

“…To reduce transverse tubule [Ca 2ϩ ] to negligible levels in our conditions requires 1 second. 18,29,30 This does not affect our conclusions here, because resting Ca 2ϩ sparks are not due to Ca 2ϩ influx, CaSpF is unaltered in 0Ca-0Na/EGTA, and all comparisons are at times Ͼ2 seconds. 18 Nevertheless, this geometric constraint limits temporal resolution such that we can only claim BayK effects on CaSpF as maximal in Ͻ10 seconds (for both 50 and 500 nmol/L BayK).…”

Cardiac T-Tubule Microanatomy and Function

“…One of the functional attributes of t-tubules is the presence of slow diffusion zones close to t-tubule sarcolemma (164,189,209). Restricted diffusion occurs both at extracellular and intracellular spaces near the t-tubule membrane with accumulated ions such as calcium, which was originally observed as a theoretical "fuzzy space" (119).…”

Ca2+ currents in cardiac myocytes: Old story, new insights

“…An alternative approach has been to use the diffusion delay between changing the bulk extracellular solution and the change in the ttubules. In guinea pig ventricular myocytes, a rapid change of extracellular [Ca 2+ ] leads to a biphasic change in I Ca , 36% of I Ca changing rapidly (~20 ms time constant) and 64% slowly (~200 ms) (Shepherd and McDonough, 1998). This indicates ~64% of functioning Ca 2+ channels are in the t-tubules.…”

“…This indicates ~64% of functioning Ca 2+ channels are in the t-tubules. In atrial myocytes, which lack t-tubules, changing the bathing Ca 2+ produced a monophasic rapid changes of I Ca (Shepherd and McDonough, 1998).…”