Dexamethasone and cardiac potassium currents in the diabetic rat

Abstract

Experiments were designed to compare effects of dexamethasone on transient (Ipeak) and sustained (Isus) K+ currents in control and diabetic rat myocytes. Ventricular myocytes were isolated from control or type 1 streptozotocin (STZ)-induced diabetic male and female rats. Currents were measured using whole-cell voltage-clamp methods. Incubation of cells from control males or females with 100 nM dexamethasone (5-9 h) significantly (P<0.005) augmented Isus (by 28-31%). Ipeak was unchanged. Isus augmentation was abolished by cycloheximide or cytochalasin D, but not by inhibition of protein kinases A or C. Inhibition of tyrosine kinases by genistein (but not its inactive analog genistin) prevented the increase of Isus by dexamethasone. In marked contrast, dexamethasone had a significantly (P<0.015) smaller effect on Isus (11% increase) in cells from male STZ-diabetic rats, as compared to control cells. However, Isus augmentation in cells from female STZ-diabetic rats was normal (31% increase). In ovariectomized-diabetic rats, Isus was unchanged by dexamethasone. The reduced effect in diabetic males might be due to preactivation of tyrosine kinases linking dexamethasone to current modulation. In conclusion, type I diabetes is associated with gender-specific changes in sensitivity of K+ currents to glucocorticoids, linked to alterations in tyrosine-phosphorylated proteins.

Figure 1

Effects of dexamethasone (100 nM) on outward K⁺ currents in rat ventricular myocytes. (a) Sample current traces obtained from male control rats in the absence (left) or after 6 h in 100 nM dexamethasone (right). Currents are in response to 500 ms pulses from −80 mV to potentials ranging from −10 to +50 mV. (b) Mean current densities as a function of membrane potentials (circles in the absence, squares in the presence of 100 nM dexamethasone, 5–8 h). The current–voltage relationships for I_peak (left) and I_sus (right, measured at 500 ms) show that only I_sus is altered, with significant augmentation (P<0.005, denoted by ^** here and in subsequent figures) between −30 and +50 mV.

Figure 2

Cycloheximide and cytochalasin D abolish the augmentation of I_sus by dexamethasone. Mean (±s.e.m.) I_sus density at +50 mV is shown in the absence of drugs, with dexamethasone alone (100 nM, 5–8 h), and with dexamethasone and either cycloheximide (2 μM) or cytochalasin D (1 μM). These were added 1 or 2 h before dexamethasone, respectively.

Figure 3

Effects of dexamethasone are maintained in the presence of PKC and PKA inhibition. (a) Sample current traces (same protocol as Figure 1) obtained from two cells, in the absence of dexamethasone (left), or (on the right) following 5 h in (100 nM) dexamethasone and the PKC inhibitor bis-indolylmaleimide (100 nM, added 30 min before dexamethasone). (b) Mean I_sus densities (at +50 mV) in the absence of drugs (open bars) or with dexamethasone and the PKC inhibitor (left, hatched bar) or (on the right) with dexamethasone and the PKA inhibitor RpCAMPS (100 μM) added 30 min before dexamethasone (hatched bar).

Figure 4

Genistein inhibits the augmentation of I_sus by dexamethasone. (a) Sample current traces, obtained in response to pulses from −80 to potentials ranging from 0 to +50 mV, in an untreated cell (left), a cell treated (5 h) with 100 nM dexamethasone (center) and a cell treated with dexamethasone (for 5.5 h) and 100 μM genistein, added 30 min before dexamethasone (right). (b) Summary data showing mean current densities at +50 mV in the absence or presence of dexamethasone, and with dexamethasone and either genistein (100 μM) or the inactive analog genistin (100 μM). Whereas genistein significantly (P<0.005) blunts I_sus augmentation by dexamethasone, genistin has no effect, suggesting that tyrosine kinases mediate the effects on I_sus.

Figure 5

Absence of I_sus augmentation by dexamethasone in cells from STZ-diabetic males. (a) Sample current traces (voltage steps from −80 to potentials ranging from −10 to +50 mV) in two cells from diabetic rats, in the absence (left) or after 7 h in (100 nM) dexamethasone (right). (b) Current–voltage relationships for I_peak (left) and I_sus (right), showing no significant differences in mean current densities in the absence (circles) or presence (5–9 h) of dexamethasone (squares).

Figure 6

Augmentation of I_sus by dexamethasone in STZ-diabetic females. (a) Sample current traces (in response to −10 to +50 mV voltage steps) in two cells from diabetic females, either untreated (left) or following 5.5 h in (100 nM) dexamethasone. (b) Current–voltage relationships for I_peak (left) and I_sus (right), showing the augmentation of I_sus, which was highly significant (P<0.005) between −10 and +50 mV.