[Measurements of 45 Ca-uptake of intestinal smooth muscle and the hypothesis of Ca-spikes]

Calcium currents of cesium loaded isolated smooth muscle cells (urinary bladder of the guinea pig)

Single smooth muscle cells isolated from the urinary bladder of the guinea-pig were studied at 35 degrees C in a solution composed of 150 mM NaCl, 3.6 mM CaCl2, 1.2 mM MgCl2, 5.4 mM KCl, 20 mM TEA-Cl, 5 mM glucose, 10 mM HEPES/NaOH (pH 7.4). Whole cells were clamped with a single patch electrode. The clamp settled a step from -65 to -5 mV within 260 microseconds, and afterwards the voltage inhomogeneities were less than 2 mV (measured at the cell edge with a second electrode). The calcium inward current iCa was dissected from net currents by blocking potassium outward currents by means of patch electrodes filled with 130 mM CsCl (Klöckner and Isenberg 1985 a). Pyruvate, succinate and oxalacetate in the patch electrode stabilized iCa and prevented its "run down". 140 ms long clamp steps from -65 to -5 mV evoked a net inward current which could be reversibly blocked by 5 mM NiCl2. The "Ni-sensitive" difference current iCa peaked within 2-4 ms to about 1 nA per cell. Afterwards it completely inactivated; the inactivation could be fitted with three exponentials (time constants of 4, 30, and 250 ms, respectively). The half decay time of 16 ms suggests that most of the inactivation resulted from the fast exponential process. The reference current in the presence of Ni was nearly time independent and almost zero; therefore, iCa could be approximated from the net inward current using the zero current as a reference line.(ABSTRACT TRUNCATED AT 250 WORDS)

[The myogenic basis of smooth muscle motility (author's transl)]

Many mammalian smooth muscle tissues are able to produce spontaneous, myogenic activity. Five types of phasic-rhythmic activity can be distinguished: 1) Spikes: brief depolarizations of the membrane which trigger calcium release and contraction; 2) oscillations of the membrane potential of the second-rhythm type (SR) generating the spikes; 3) various organ-specific rhythms such as gastric and ureteral peristalsis which can be grouped together as basic organ-specific rhythms (BOR); 4) slower fluctuations of the minute-rhythm type (MR); 5) an hour-rhythm (HR) as the slowest type. In addition, some tissues generate tonic activity by special processes which can operate without spike discharges of the cell membrane. A selective blockade of phasic and tonic components is possible with some members of the group of so-called calcium antagonists. This indicates that two different calcium activation systems exist in the membrane of smooth muscle cells (P- and T-systems). Selective P- and T-blockade offers new possibilities for pharmacological influences on the smooth muscle system.