Inward calcium currents in cultured and freshly isolated detrusor muscle cells: evidence of a T-type calcium current.
OBJECTIVE
We carefully examined the possible routes of Ca2+ influx, and determined whether cultured cells retain Ca2+ channels and whether the culturing process changes their properties.


METHODS
Inward currents were measured under voltage clamp in freshly isolated cells and myocytes from confluent cell cultures of detrusor smooth muscle.


RESULTS
In guinea pig and human cells mean peak inward current density plus or minus standard deviation decreased significantly in cell culture (2.0 +/- 0.9 versus 4.5 +/- 2.2 pA.pF.(-1)) but there was no species variation. In primary cultured and passaged guinea pig cells an inward current was identified as L-type Ca2+ current. In freshly isolated cells another component to the inward current was identified that was insensitive to 20 micromol. l(-1) verapamil and 20 to 50 micromol. l(-1) cadmium chloride but abolished by 100 micromol. l(-1) nickel chloride and identified as T-type Ca2+ current. In addition, total inward current was greater at a holding potential of -100 than -40 mV., also indicating a component of current activated at negative voltage. Steady state activation and inactivation curves of the net inward current were also compatible with a single component in cultured cells but a dual component in freshly isolated cells. The action potential was completely abolished in cultured cells by L-type Ca2+ channel blockers but incompletely so in freshly isolated cells. Outward current depended strongly on previous inward current, suggesting a predominant Ca2+ dependent outward current.


CONCLUSIONS
In freshly isolated guinea pig cells T and L-type Ca2+ current is present but T-type current is absent in confluent cultures.
Sui
G P
GP
Institute of Urology and Nephrology, London, United Kingdom.
Wu
C
C
Fry
C H
CH
D000818:Animals; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D002478:Cells, Cultured; D006168:Guinea Pigs; D006801:Humans; D009130:Muscle, Smooth / Q000166:cytology* / Q000502:physiology*; D001743:Urinary Bladder / Q000166:cytology* / Q000502:physiology*
D015220:Calcium Channels; D002118:Calcium