SUMMARY1. ATP-activated inward current in PC12 pheochromocytoma cells was characterized using the whole-cell voltage-clamp technique.2. ATP (100 4aM) applied extracellularly elicited an inward rectifying current with a reversal potential of about + 7 mV. The current was desensitized in seconds in spite of continued presence of ATP.3. A comparison was made of the ability of ATP and its analogues. The order of potency in activating the inward current was ATP > ATPyS > ADP; AMP, adenosine and a,,-methylene ATP were inactive at concentrations up to 1 mM.4. The ATP-activated current was also observed when external Na+ and Ca2+ were replaced with K+, TEA, Tris or glucosamine. The order of ion selectivity was Na+ > K+ > TEA * Tris > glucosamine.5. The ATP-activated current was also recorded in extracellular solutions containing Ca2+ as the only external cation. The amplitude increased as the concentration of Ca2+ was increased in the range between 1-8 and 16-2 mm. However, the current amplitude decreased at higher Ca2+ concentrations and the current was not recorded in 110 mM-Ca2+ solution. 6. In the presence of 140 mM-Na+ in the external solution, the current amplitude also decreased as the external Ca2+ concentration was increased (from 1-8 to 16-2 mM).7. The results indicate that Ca2+ as well as monovalent cations permeate through the ATP-sensitive pathway and that Ca2+ blocks ion permeation, including its own permeation through the pathway. This regulation by extracellular Ca2+ is different to the ATP-activated current in smooth muscle cells.

Cardiac-specific overexpression of murine cardiac calsequestrin results in depressed cardiac contractile parameters, low Ca 2؉ -induced Ca 2؉ release from sarcoplasmic reticulum (SR) and cardiac hypertrophy in transgenic mice. To test the hypothesis that inhibition of phospholamban activity may rescue some of these phenotypic alterations, the calsequestrin overexpressing mice were cross-bred with phospholamban-knockout mice. Phospholamban ablation in calsequestrin overexpressing mice led to reversal of the depressed cardiac contractile parameters in Langendorff-perfused hearts or in vivo. This was associated with increases of SR Ca 2؉ storage, assessed by caffeine-induced Na ؉ -Ca 2؉ exchanger currents. The inactivation time of the L-type Ca 2؉ current (I Ca ), which has an inverse correlation with Ca 2؉ -induced SR Ca 2؉ release, and the relation between the peak current density and half-inactivation time were also normalized, indicating a restoration in the ability of I Ca to trigger SR Ca 2؉ release. The prolonged action potentials in calsequestrin overexpressing cardiomyocytes also reversed to normal upon phospholamban ablation. Furthermore, ablation of phospholamban restored the expression levels of atrial natriuretic factor and ␣-skeletal actin mRNA as well as ventricular myocyte size. These results indicate that attenuation of phospholamban function may prevent or overcome functional and remodeling defects in hypertrophied hearts.Hypertrophy of ventricular myocardium is postulated to be an adaptive response to relative increases in external workload, induced by endocrine, paracrine, autocrine, and mechanical factors or decreased myocardial contractility (1). The increase in heart mass has been implicated to normalize cardiac function by decreasing wall stress. However, a sustained imbalance between workload and muscle contractility may lead to progressive thinning of the left ventricular wall and chamber dilation associated with decompensated hypertrophy and heart failure (2, 3). Studies in human and animal models have shown that cardiac hypertrophy is associated with impaired sarcoplasmic reticulum (SR) 1 Ca 2ϩ modulation, leading to aberrant cardiac contraction and relaxation (4 -8). Although several Ca 2ϩ -related signaling molecules, such as calcineurin, Ca 2ϩ -calmodulin kinase, and Ca 2ϩ

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1. Effects of substance P (SP) and other tachykinins on membrane currents were investigated using whole cell voltage clamp in cultured sensory neurons isolated from rat dorsal root ganglia. 2. SP (100 nM) evoked an inward current in two-thirds of the cells at negative potentials. In most of the cells that generated the inward current in response to SP, capsaicin also activated an inward current. The SP-evoked inward current was not observed in cells loaded with 2 mM guanosine 5'-O-(2-thiodiphosphate) (GDP beta S). 3. Neurokinin A (NKA) or neurokinin B (NKB) also activated an inward current. At 100 nM of each agonist, the order was NKB > NKA > SP with respect to activated current amplitude. 4. The tachykinin-activated current was reversed around +10 mV with a standard extracellular solution containing 140 mM NaCl. The reversal potential became more negative when extracellular NaCl was reduced by substituting with sucrose. The inward current was also activated in cells bathed in an extracellular solution containing Cs+, tetraethylammonium (TEA) or N-methyl-D-glucamine (NMDG) as a major cation instead of Na+. The order of permeability, determined from the reversal potential of the current, was Cs+ not equal to Na+ > TEA > NMDG. The amplitude of the inward current activated by NKB was increased when extracellular Na+ was replaced with Cs+, TEA or NMDG. 5. Permeability of Ca2+ was tested using an extracellular solution containing Ca2+ as the only cation (111.8 mM Ca2+ outside). Under this condition, NKB evoked an inward current that reversed around +30 mV. 6. The results suggest that SP and other tachykinins activate nonselective cation channels, which are also permeable to Ca2+, through receptors which are more responsive to NKB than to SP or NKA. The channel activation may serve as a mechanism underlying tachykinin-mediated excitatory neurotransmission in sensory neurons.

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