Inward-rectifier K+ current in guinea-pig ventricular myocytes exposed to hyperosmotic solutions

Osmolality- and Na+-dependent effects of hyperosmotic NaCl solution on contractile activity and Ca2+ cycling in rat ventricular myocytes

Hypertonic NaCl solutions have been used for small-volume resuscitation from hypovolemic shock. We sought to identify osmolality- and Na(+)-dependent components of the effects of the hyperosmotic NaCl solution (85 mOsm/kg increment) on contraction and cytosolic Ca(2+) concentration ([Ca(2+)](i)) in isolated rat ventricular myocytes. The biphasic change in contraction and Ca(2+) transient amplitude (decrease followed by recovery) was accompanied by qualitatively similar changes in sarcoplasmic reticulum (SR) Ca(2+) content and fractional release and was mimicked by isosmotic, equimolar increase in extracellular [Na(+)] ([Na(+)](o)). Raising osmolality with sucrose, however, augmented systolic [Ca(2+)](i) monotonically without change in SR parameters and markedly decreased contraction amplitude and diastolic cell length. Functional SR inhibition with thapsigargin abolished hyperosmolality effects on [Ca(2+)](i). After 15-min perfusion, both hyperosmotic solutions slowed mechanical relaxation during twitches and [Ca(2+)](i) decline during caffeine-evoked transients, raised diastolic and systolic [Ca(2+)](i), and depressed systolic contractile activity. These effects were greater with sucrose solution, and were not observed after isosmotic [Na(+)](o) increase. We conclude that under the present experimental conditions, transmembrane Na(+) redistribution apparently plays an important role in determining changes in SR Ca(2+) mobilization, which markedly affect contractile response to hyperosmotic NaCl solutions and attenuate the osmotically induced depression of contractile activity.

Insensitivity of cardiac delayed-rectifier I(Kr) to tyrosine phosphorylation inhibitors and stimulators

1. The rapidly activating delayed-rectifying K+ current (I(Kr)) in heart cells is an important determinant of repolarisation, and decreases in its density are implicated in acquired and inherited long QT syndromes. The objective of the present study on I(Kr) in guinea-pig ventricular myocytes was to evaluate whether the current is acutely regulated by tyrosine phosphorylation. 2. Myocytes configured for ruptured-patch or perforated-patch voltage-clamp were depolarised with 200-ms steps to 0 mV for measurement of I(Kr) tail amplitude on repolarisations to -40 mV. 3. I(Kr) in both ruptured-patch and perforated-patch myocytes was only moderately (14-20%) decreased by 100 microM concentrations of protein tyrosine kinase (PTK) inhibitors tyrphostin A23, tyrphostin A25, and genistein. However, similar-sized decreases were induced by PTK-inactive analogues tyrphostin A1 and daidzein, suggesting that they were unrelated to inhibition of PTK. 4. Ruptured-patch and perforated-patch myocytes were also treated with promoters of tyrosine phosphorylation, including phosphotyrosyl phosphatase (PTP) inhibitor orthovanadate, exogenous c-Src PTK, and four receptor PTK activators (insulin, insulin-like growth factor-1, epidermal growth factor, and basic fibroblast growth factor). None of these treatments had a significant effect on the amplitude of I(Kr). 5. We conclude that Kr channels in guinea-pig ventricular myocytes are unlikely to be regulated by PTK and PTP.